referring to fig1 and 2 an air filtering device 10 is shown adapted to operate vertically by drawing air longitudinally therethrough from an intake 12 , filtering the air as it travels downwardly through the device 10 , and expelling the air through the outlet ports 14 adjacent the base of the device 10 . air is drawn through the device by a fan 16 driven by an electric motor 18 . an electric conduit 20 extends from the electric motor 18 out of the device 10 to a source of electricity for providing power to the motor 18 . the fan 16 and motor 18 are mounted at the base of the device 10 in a lower fan enclosure member 22 of unitary construction having a base 24 and a generally cylindrical longitudinally extending lateral wall 26 . the outlets 14 for exhausting air from the device 10 are located in this lateral wall 26 . the lateral wall 26 has a laterally extending annular portion 28 adjacent the top thereof from the midportion of which projects a longitudinally extending annular portion 30 of the wall 26 . the lateral portion 28 and the longitudinal portion 30 form a first annular or circular shoulder 32 on the inside of the lateral wall 26 and a second annular or circular shoulder 34 on the outside of the lateral wall 26 adjacent the first shoulder 32 and laterally spaced exteriorly of the first shoulder 32 , the two shoulders 32 , 34 being separated by the longitudinally extending annular portion 30 . both shoulders 32 , 34 lie in planes generally perpendicular to the longitudinal axis of the device 10 and face in the same longitudinal direction . the planes in which the shoulders 32 , 34 lie are parallel and slightly spaced . interlocked with the lower fan enclosure member 22 is an upper fan enclosure member 36 of unitary construction which together with the lower fan enclosure member 22 encases the fan 16 and electric motor 18 . referring to fig2 through 5 the upper fan enclosure member 36 has a flat top surface 38 disposed in a plane generally perpendicular to the longitudinal axis of the device 10 and has therein a plurality of air passages 40 for the passage of air therethrough . the air passages 40 are arranged in a rectangular grid formed by vanes 42 . the vanes 42 are disposed parallel to the longitudinal axis of the device 10 to assist in guiding the longitudinal flow of the air being drawn through the device 10 . the upper fan enclosure member 36 also includes a generally cylindrical longitudinally extending lateral wall 44 extending downwardly from the top surface 38 to annular members 46 and 48 . the annular members 46 extend laterally from the lateral wall 44 adjacent the bottom thereof forming generally annular or circular shoulders 50 lying in a plane generally perpendicular to the longitudinal axis of the device 10 in a manner similar to that of the first shoulder 32 of the lower fan enclosure member 22 . the annular shoulders 50 are opposed to the first shoulder 32 and are adapted for mating with the first shoulder 32 . the annular member 48 extends longitudinally from the lateral wall 44 and serves as a guide in joining the upper fan enclosure member 36 with the lower fan enclosure member 22 and as a reinforcing member in the interengagement of the two enclosure members 22 , 36 by coming into sliding engagement with the interior of the lateral wall 26 adjacent the laterally extending portion 28 of the lateral wall 26 . the upper fan enclosure member 36 and the lower fan enclosure member 22 include members for detachably locking or latching the enclosure members together . extending longitudinally downward from the bottom of the lateral wall 44 of the upper fan enclosure member 36 are at least two longitudinally extending locking or latching members 52 of limited thickness . the locking members 52 are incorporated in the annular member 48 for part of their respective lengths and have a thickness less than the thickness of the lateral wall 44 whereby they are rendered semi - rigid and capable of being bent laterally . adjacent the tip of each member 52 is a lateral protrusion or bead 54 facing outwardly on each locking member 52 . when the upper fan enclosure member 36 and the lower fan enclosure member 22 are in engagement with their shoulders 32 , 50 mated in opposed contact the lateral protrusions 54 of the locking members 52 engage the upper surfaces 56 of some of the outlet ports 14 thereby locking the lower fan enclosure member 22 and upper fan enclosure member 36 together . alternatively , separate openings may be provided in the lateral wall 26 for the purpose of engaging the locking members 52 . the two enclosure members 22 , 36 may be detached from one another by bending the locking members 52 inwardly out of engagement with the surfaces 56 . in bringing the upper fan enclosure member 36 and the lower fan enclosure member 22 into engagement the locking members 52 are bent inwardly as the protrusions 54 slide over the interior surface of the lateral wall 26 of the lower fan enclosure member 22 until the protrusions 54 reach the outlet ports 14 at which point the locking members 52 snap into their rest positions locking the two enclosure members 22 , 36 together . referring to fig2 and 7 through 15 , positioned above the upper fan enclosure member 36 in the device 10 is a filter 58 for filtering the air being conveyed through the device 10 . the filter 58 has an elliptical disc shape formed by a filter material enclosure 60 having first and second planar spaced parallel elliptical faces 62 , 64 joined about their perimeters by a peripheral wall 66 perpendicular to the two faces 62 , 64 . each elliptical face 62 , 64 has therein a plurality of openings 68 , 70 for the passage of air through the filter 58 . the filter material enclosure 60 comprises an upper unitary piece 72 and a lower unitary piece 74 . the upper piece 72 includes the elliptical first face 62 and a peripheral flange 76 extending perpendicularly from the elliptical first face 62 . about the periphery of the first face 62 between the peripheral flange 76 and the plurality of openings 68 is a border strip 78 having no openings therein . projecting from the first face 62 at this border 78 in the same direction as the peripheral flange 76 are a plurality of projections 80 distributed about the periphery of the upper piece 72 . the lower piece 74 comprises the second planar elliptical face 64 and a peripheral flange 82 located adjacent the periphery of the second face 64 and projecting therefrom perpendicularly with respect to the plane of the second face 64 . the flange 82 is slightly indented from the very edge of the second face 64 by a distance approximately equal to the thickness of the peripheral flange 76 of the upper piece 72 . the lower piece flange 82 is approximately equal in height to the upper piece flange 76 . similar to the border 78 in the upper piece 72 , the lower piece 74 has in its face 64 at the periphery thereof between the plurality of openings 70 and the flange 82 a border strip 84 having no openings therethrough . situated on this border strip 84 about the periphery of the lower piece 74 are a plurality of socket members 86 projecting in the same direction as the peripheral flange 82 to which they are joined at their outside lateral edges . each socket member 86 has an opening 88 therein . the upper piece 72 and the lower piece 74 are adapted to be joined by means of the peripheral flanges 76 and 82 and the socket members 86 and corresponding projections 80 . the inside face 90 of the peripheral flange 76 is adapted to come into sliding engagement with the outside face 92 of the peripheral flange 82 to form thereby the peripheral wall 66 of the filter material enclosure 60 . the end extremity of the peripheral flange 76 abuts the fringe of the planar face 64 extending beyond the flange 82 . the projections 80 are received by the socket openings 88 in a friction fit similar to that between the flanges 76 and 82 so that the upper and lower pieces 72 , 74 are firmly joined together with their junction reinforced by the interengagement of the spaced socket openings 88 and projections 80 . the joined upper and lower pieces 72 , 74 thereby form the filter material enclosure 60 . contained within the filter material enclosure 60 is filter material 94 comprising a layer of thermoplastic foam 96 adjacent the inside of the first face 62 and a layer of activated charcoal 98 adjacent the inside of the second face 64 . the sizes of the openings 68 , 70 in the respective faces 62 , 64 are dimensioned so as to retain the filter material 94 . referring to fig2 and 5 , a recess 100 is located in the upper fan enclosure member 36 at the juncture of the top surface 38 and the lateral wall 44 of the upper fan enclosure member 36 . projecting from the top surface 38 of the upper fan enclosure member 36 adjacent the recess 100 are two longitudinally extending shoulders 102 each having an edge 104 which together with a planar surface 106 of the recess 100 define a plane at an angle with respect to the longitudinal axis of the device 10 . the recess 100 and the shoulders 102 are adapted to receive and hold in position the filter 58 at an angle with respect to the longitudinal axis of the device 10 and thereby at the same angle with respect to the general direction of flow of air through the device 10 . the causes 100 and the shoulders 102 receive that portion of the filter 58 adjacent the peripheral wall 66 of the filter 58 at one of the points where the major axis of the ellipse formed by the filter 58 passes through the peripheral wall 66 . projecting laterally from the peripheral wall 66 of the filter 58 is a tab 108 situated at the point where the major axis of the ellipse formed by the filter 58 passes through the peripheral wall 66 . the tab 108 is a unitary projection of the second filter face 64 and forms a part of the lower filter piece 74 . the filter 58 is held in position by this tab 108 which is inserted into a slot 110 in the upper fan enclosure member 36 located adjacent the base of the recess surface 106 . in this position the peripheral wall 66 of the filter 58 abuts against a second recess surface 112 perpendicular to the first recess surface 106 and having a contour generally identical to the elliptical shape of the periphery of the filter 58 . with reference to fig1 and 3 , enclosing the filter 58 and the upper fan enclosure member 36 and holding the filter 58 in position on the upper fan enclosure member 36 is a housing 114 having a top surface 116 and a generally cylindrical longitudinally extending lateral wall 118 . the wall 118 terminates in an annular mating shoulder 120 lying in a plane generally perpendicular to the longitudinal axis of the device 10 . this shoulder 120 is adapted to engage the second mating shoulder 34 of the lower fan enclosure member 22 and for this purpose is opposed to the second mating shoulder 34 and faces in the longitudinal direction toward the base of the device 10 . the mating shoulder 120 is circular in configuration . the lateral wall 118 of the housing 114 has at the mating shoulder 120 a diameter equal to that of the second mating shoulder 34 of the lower fan enclosure member 22 and slightly greater than the diameter of the longitudinally extending annular portion 30 of the lateral wall 26 whereby the housing 114 is adapted to engage the lower fan enclosure member 22 by its circular mating shoulder 120 coming into abutting contact with the second mating shoulder 34 and with the inside surface of its lateral wall 118 adjacent the shoulder 120 coming into overlapping sliding engagement with the outside surface of the annular portion 30 of the lateral wall 26 of the lower fan enclosure member 22 to provide reinforcement to the interengagement of the lower fan enclosure member 22 and the housing 114 . with this arrangement the housing 114 , having a diameter greater than that of the upper fan enclosure member 36 , is positioned concentrically with respect to the upper fan enclosure member 36 which it encloses . the housing 114 and the lower fan enclosure member 22 include members for detachably locking or latching themselves together . projecting longitudinally and upwardly from the annular portion 30 of the lateral wall 26 of the lower fan enclosure member 22 are two flexible arm members 122 , 124 which each have a portion of reduced thickness with respect to the thickness of the annular portion 30 thereby rendering them semi - rigid and flexible . adjacent the end of each arm member 122 , 124 is a laterally projecting bead member or protrusion 126 . each lateral protrusion 126 is positioned to engage an opening 128 in the lateral wall 118 of the housing 114 when the housing 114 and the lower fan enclosure member 22 are in engagement with their mating shoulders 34 , 120 in opposed contact . in bringing the housing 114 and the lower fan enclosure member 22 into engagement the flexible arm members 122 , 124 are bent inwardly as the lateral protrusions 126 slide over the interior surface of the lateral wall 118 of the housing 114 until the protrusions 126 reach the openings 128 at which point the flexible arm members 122 , 124 snap into their rest positions locking the housing 114 and the lower fan enclosure member 22 together . the housing 114 and the lower fan enclosure member 22 may be detached from one another by pressing the protrusions 126 radially inward through the openings 128 while simultaneously lifting the housing 114 away from the lower fan enclosure member 22 . located in the top surface 116 of the housing 114 is the intake 12 for the device 10 . the intake 12 comprises a plurality of ports 130 in the top surface 116 separated by a series of parallel louver boards 132 . the top surface 116 is generally planar and has an elliptical shape corresponding generally to the elliptical configuration of the filter 58 and is disposed at an angle with respect to the longitudinal axis of the device 10 equal to the angle assumed by the filter 58 when positioned in the recess 100 and on the shoulders 102 of the upper fan enclosure member 36 . the housing 114 is arranged so that when in engagement with the lower fan enclosure member 22 and radially positioned so as to be locked thereto by means of the flexible arm members 122 , 124 and their protrusions 126 , the top surface 116 lies parallel to and immediately adjacent the first face 62 of the filter 58 whereby the filter 58 is retained in position and the plurality of ports 130 lie parallel to and immediately adjacent the filter 58 . the louver boards 132 are disposed in the top surface 116 so that when the housing 114 is in this configuration , the louver boards 132 are disposed parallel to the longitudinal axis of the device 10 and thereby tend to direct inflowing air longitudinally through the device 10 and tend to direct air flow through the filter 58 in a general direction at an angle with respect to the plane of the filter 58 complementing the angle at which the filter 58 is disposed with respect to the longitudinal axis of the device 10 . the elliptical shape of the filter 58 is selected with respect to the angle at which the filter 58 will lie with respect to the longitudinal axis of the device 10 so that the periphery of the filter 58 will extend to the interior of the generally circular lateral wall 118 of the housing 114 all around the periphery of the filter 58 thereby minimizing air flow around the filter 58 . referring to fig1 through 20 , a modified form 134 of an air filtering device according to the invention is shown . most of the components of the modified air filtering device 134 are similar to or identical to the corresponding components in the air filtering device 10 . these include the fan and electric motor ( not shown ), lower fan enclosure member 136 , upper fan enclosure member 138 , and an upper housing member 140 , each of unitary construction . the structure of the filter 142 also is identical to the filter 58 of device 10 . however , the device 134 includes three such filters having also filters 144 and 146 identical in structure to the filter 142 . the lowermost filter 142 is received and positioned in the upper fan enclosure member 138 in a manner identical to that in the device 10 at an angle with respect to the longitudinal axis of the device 134 and the general direction of air flow through the device 134 . the second and third filters 144 , 146 are positioned parallel to the first filter 142 and are staggered in relation to one another and with respect to the filter 142 so that their respective peripheries are longitudinally aligned defining a cylinder in overall configuration . the uppermost filter 146 in a manner similar to that of the filter 58 in device 10 is parallel to and immediately adjacent the inlet 148 in the top surface of the housing 140 . the filters 144 and 146 are in a manner identical to that for filter 142 thereby disposed at an angle with respect to the longitudinal axis of the device 134 and the general direction of air flow through the device 134 equal to the angle of the filter 142 . to accomodate the two additional filters 144 , 146 the air filtering device 134 includes in addition to the upper housing member 140 an intermediate housing member 150 of unitary construction positioned between the lower fan enclosure member 136 and the upper housing member 140 to increase the space within the device 134 between the upper fan enclosure member 138 and the air inlet 148 . the intermediate housing member 150 has a generally cylindrical lateral wall 152 and a generally annular or circular mating shoulder 154 on its lateral wall 152 at the upper longitudinal end thereof lying in a plane generally perpendicular to the longitudinal axis of the device 134 . the annular shoulder 154 is opposed to the annular shoulder 156 of the upper housing member 140 and is adapted for mating therewith by coming into abutting contact to interengage the intermediate housing member 150 and the upper housing member 140 . the lateral wall 152 of the intermediate housing member 150 includes an annular portion 158 extending longitudinally upward beyond the mating shoulder 154 . the wall of the upper housing member 140 has at its mating shoulder 156 a diameter equal to that of the mating shoulder 154 and slightly greater than the diameter of the longitudinally extending portion 158 whereby when the upper housing member 140 and the intermediate housing member 150 are interengaged with their respective mating shoulders 154 , 156 in opposed abutting contact , the inside surface of the lateral wall 160 of the upper housing member 140 adjacent the mating shoulder 156 comes into overlapping sliding engagement with the outside surface of the annular portion 158 of the lateral wall 152 of the intermediate housing member 150 to provide reinforcement to the interengagement of the upper housing member 140 and the intermediate housing member 150 . the structure for interengaging the upper housing member 140 and intermediate housing member 150 is therefore similar to the structure for interengaging the upper housing member 140 and the lower fan enclosure member 136 . in the absence of the intermediate housing member 150 , the upper housing member 140 may be engaged directly to the lower fan enclosure member 136 with mating shoulder 156 of the upper housing member 140 in opposed abutting contact with the annular mating shoulder 162 of the lower fan enclosure member . two longitudinally extending flexible arm members 164 are also provided for detachably interlocking or interlatching the upper housing member 140 and the intermediate housing member 150 . these members 164 have laterally extending beads or protrusions 166 thereon which cooperate with a corresponding number of laterally extending openings 168 in the wall 160 of the upper housing member 140 in a manner similar to that of the locking arm members 122 , 124 and openings 128 of the housing 114 and the lower fan enclosure member 22 of the device 10 . members are also provided for interengaging the intermediate housing member 150 and the lower fan enclosure member 136 . the upper housing member 140 increases gradually in diameter progressing from top to bottom thereby creating a more aesthetically pleasing appearance . the intermediate housing member 150 similarly increases gradually in diameter from top to bottom having a diameter towards the top thereof generally equal to the diameter at the base of the upper housing member 140 . since the wall 160 at the base of the upper housing member 140 is of a size adapted for engagement with the annular shoulder 162 of the lower fan enclosure member 136 , the lateral wall 152 of the intermediate housing member 150 is necessarily greater in diameter towards its base than the diameter of the annular shoulder 162 of the lower fan enclosure member 136 . accordingly , longitudinally extending parallel ribs 172 are situated on the inside surface of the lateral wall 152 of intermediate housing member 150 terminating slightly above the lower end of the intermediate housing member 150 in shoulders 174 . the shoulders 174 are disposed in a plane generally perpendicular to the longitudinal axis of the device 134 opposed to the annular shoulder 162 of lower fan enclosure member 136 and are adapted to engage the annular shoulder 162 by coming into abutting contact with the annular shoulder 162 . to reinforce the interengagement of the intermediate housing member 150 and the lower fan enclosure member 136 the longitudinally extending edge 176 of each rib 172 comes into overlapping sliding engagement with the outside surface of the longitudinally extending annular portion 178 of the lateral wall 180 of lower fan enclosure member 136 . similar to the structure of the lateral wall 26 of lower fan enclosure member 22 , the lateral wall 180 of lower fan enclosure member 136 also includes a laterally extending portion 182 which together with the annular portion 178 defines the annular shoulder 162 of lower fan enclosure member 136 . the laterally extending portion 182 ends in an outside edge 184 which also provides reinforcement to the interengagement of the intermediate housing member 150 and the lower fan enclosure member 136 . when the intermediate housing member 150 and the lower fan enclosure member 136 are interengaged , the outside edge 184 , forming a circular laterally projecting flange , abuts the inside surface of the lateral wall 152 of the intermediate housing member 150 immediately adjacent the base of the intermediate housing member 150 . for detachably locking or latching the lower fan enclosure member 136 and the intermediate housing member 150 together , two locking ribs 186 are located on the lateral wall 152 of the intermediate housing member 150 between pairs of closely spaced longitudinally extending parallel ribs 172 . each locking rib 186 lies in a plane generally perpendicular to the longitudinal axis of the device 134 . extending longitudinally from the lower fan enclosure member 136 are flexible arm members 188 corresponding in number and position to the locking ribs 186 . the flexible arm members 188 are similar in construction to the arm members 122 , 124 of lower fan enclosure member 22 and function to a similar manner to lock the intermediate housing member 150 to the lower fan enclosure member 136 by coming into overlapping opposed engagement with the locking ribs 186 . so that the arm members 188 will deflect laterally inwardly as the intermediate housing member 150 is brought into engagement with the lower fan enclosure member 136 the lateral protrusions 190 on the arm members 188 each have sloping top surfaces 192 which cooperate with sloping bottom surfaces 194 on the locking ribs 186 . an alternative mechanism for detachably interlocking the lower fan enclosure member 136 and the intermediate housing member 150 is shown for lower fan enclosure member 136 &# 39 ; and intermediate housing member of 150 &# 39 ; in fig2 through 23 in which locking ribs 186 are omitted . the outside surface of the longitudinally extending annular portion 178 &# 39 ; and the outside edge 184 &# 39 ; of the laterally extending portion 182 &# 39 ; on the one hand and the longitudinally extending edge 176 &# 39 ; of each rib 172 &# 39 ; and the inside surface of the lateral wall 152 &# 39 ; immediately adjacent the base of the intermediate housing member 150 &# 39 ; on the other hand are built to such dimensions that the lower fan enclosure member 136 &# 39 ; and the intermediate housing member 150 &# 39 ; are force fitted together and interlocked by the frictional interengagement of the outside surface of the longitudinally extending annular portion 178 &# 39 ; with the edges 176 &# 39 ; and the outside edge 184 &# 39 ; with the inside surface of wall 152 &# 39 ;. projecting from the second face 64 of the filter 58 on which they are integrally formed are a plurality of spacing members or projections 196 each extending generally perpendicularly with respect to the plane of the second face 64 and of the filter 58 . the spacing members 196 are uniform in height and each has at the end thereof one or more shoulders 198 disposed perpendicularly with respect to the spacing members 196 and parallel to the plane of the filter 58 . some of the spacing members 196 also have prongs 200 at the ends thereof . located in the first face 62 of the filter 58 are a plurality of sockets or recesses 202 similar in dimensions to the prongs 200 . each of the filters 142 , 144 , 146 has thereon spacing members 204 and sockets 206 similar to the spacing members 196 and sockets 202 of the filter 58 of the device 10 . by means of these elements the filters 142 , 144 , 146 are spaced with respect to one another with the filter 146 mounted on the filter 144 which is in turn mounted in the filter 142 . spacing member prongs 208 of one of the filters 144 , 146 similar to the spacing member prongs 200 of filter 58 are inserted into the corresponding sockets 206 in the next lower filter thereby mounting each of the filters 144 , 146 on the next lower filter with the shoulders 210 of the spacing members 204 abutting the upper face of each respective filter . the spacing members 196 and sockets 202 of the filter 58 perform no function in the structure of device 10 . neither do the spacing members 204 of filter 142 nor the sockets 206 of filter 146 . however , the structures of the devices 10 and 134 are such as to accommodate a uniform structure for the filter so that only one form of the filter need be produced regardless of the number of filters to be used in the device and their location relative to one another . consequently , the number of filters in the air filtering device of this invention may be varied with the only modification to the structure of the device being the substitution of an intermediate housing member 150 of appropriate length . all the other unitary structural elements of the air filtering device remain the same . the structures of the device 10 and the device 134 are such that access to their filters 58 , 142 , 144 and 146 is readily achieved . in device 10 , the housing 114 is detached from the lower filter enclosure member 22 by disengaging the arm members 122 , 124 from the openings 128 in the housing 114 . this can be done by pushing the arm members 122 , 124 inwardly through the spacings 128 in the housing 114 . the filter 58 can then be removed from the recess 100 and shoulders 102 and replaced if desired . in device 134 , the upper housing member 140 is disengaged from the intermediate housing member 150 by disengaging the arms 164 from openings 168 in a manner similar to that for the arm members 122 , 124 of device 10 . with the upper housing 140 removed , the intermediate housing member 150 can in turn be detached from the lower fan enclosure member 136 by bending the arm members 188 inwardly out of engagement with the latching ribs 186 . alternatively , where the intermediate housing member and the lower fan enclosure member are force fitted as with intermediate housing member 150 &# 39 ; and lower fan enclosure member 136 &# 39 ;, they may be detached by applying sufficient longitudinal force to overcome the force fit . all or any number of the filters 142 , 144 , 146 may then be disconnected from one another and removed from the device 134 and replaced . the interconnections of the filters 142 , 144 , 146 among one another are by force or friction fit via the sockets 206 and spacing members 204 having prongs 208 thereon . the stack of filters 142 , 144 , 146 is retained in position on the upper fan enclosure member 138 by the upper housing member 140 and the intermediate housing member 150 . while the various flexible locking arms have been disclosed as being on one structural member and their respective locking openings or ribs on the structural member to which the first structural member is engaged , it is posible where the structure otherwise permits for the respective locking elements to be reversed with respect to which structural member they are located on . also , while in most instances the flexible locking arms and corresponding openings or ribs have been two in number , any plural number of cooperating locking elements may be used where appropriate . each of the main structural elements of the device , i . e . the upper and lower fan enclosure members and the housing or , alternatively , the upper housing member and the intermediate housing member , is preferably a molded plastic unit . while this invention has been described as having a preferred design , it will be understood that it is capable of further modification . this application is , therefore , intended to cover any variations , uses , or adaptations of the invention following the general principles thereof and including such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains , and as may be applied to the essential features hereinbefore set forth and fall within the scope of this invention or the limits of the claims .