referring particularly to fig1 - 3 , the chair base unit comprising the present invention essentially consists of an upper supporting plate 10 to which the seat , back and footrest unit of an operatory chair , such as dental chair , may be attached for support and also to be raised and lowered to a desired position by said base unit . said base unit also comprises a flat bottom plate 12 , which is adapted to rest upon a supporting surface such as a floor , and a base plate 14 , which is attached to bottom plate 12 by a short vertical pivot 16 , for rotation of the base plate 14 and all of the mechanism supported thereby relative to the bottom plate 12 . the mechanism for moving the upper plate 10 with respect to the base plate 14 comprises the subject matter of the present invention . specifically , as will be seen from fig1 - 3 , this mechanism comprises a pair of lazy tong assemblies 18 and 20 which , as best seen from fig4 and 5 , are spaced transversely a substantial distance . each assembly , 18 and 20 , preferably comprises two pairs of crossed links 22 and 24 . each of said pairs of links 22 and 24 are pivotally connected intermediately of the ends thereof by appropriate pivot bolts 26 , which connect the upper pair of lazy tong links , and pivot bolts 28 connect the lower pair of such cross links . the lower ends of links 22 and 24 of the upper set are connected to the upper ends of said links of the lower set by additional pivot bolts 30 . the lower ends of links 24 of the lower set are pivotally connected by additional pivot bolts 32 to fixed brackets 34 , which are best shown in fig1 - 4 , the same being fixed to the upper surface of base plate 14 . correspondingly , the upper end of links 22 of the upper set are pivotally connected by additional pivot bolts 36 to brackets 38 , which are fixed to supporting plate 10 of the upper end of the base unit . the upper ends of links 24 of the upper set preferably have an anti - friction roller 40 fixed thereto for longitudinal movement within a guide channel 42 , one of such channels being disposed adjacent each side edge of the upper supporting plate 10 , as shown in fig1 - 3 and particularly in fig5 . for purposes of bracing the transversely spaced parallel sets of lazy tong assemblies 18 and 20 against lateral skewing movement or any other type of transverse movement between the upper supporting plate 10 and the base plate 14 , appropriate brace members are connected to the various links of the lazy tong assemblies . as best shown in fig5 a transverse brace bar 44 is welded at its opposite ends respectively to the upper ends of links 24 of the upper set thereof , the position of said brace bar also being illustrated in fig1 - 3 . another , somewhat u - shaped transverse brace bar 46 , extends between the lower ends of links 22 of the lower set thereof . further , still another transverse brace bar 48 , best shown in fig1 and 4 , extends between the terminal upper end of links 22 of the lower set thereof , it being understood that fig4 is an end view of the base unit as seen from the left end of fig1 . in fig4 the transverse brace bar 48 obscures a cylindrical brace bar 50 , which extends between the lower ends of the links 22 of the upper set thereof , as best shown in fig5 in which the cylindrical brace bar 50 obscures the transverse brace bar 48 , which is at the opposite end of the base from the cylindrical brace bar 50 . further , the brace bar 50 may either be tubular or solid , and in any event , the pivot bolts 30 preferably are threaded into tapped holes in the opposite ends of the cylindrical brace bar 50 . it readily can be visualized from the foregoing that the linkage structure is amply braced against lateral , skewing or slanting movement with respect to a vertical plane extending from the base of the unit and disposed between the vertically spaced sets of lazy tong assemblies . all of the various ends of the links of the several sets of lazy tong assemblies have been accounted for in the foregoing with the exception of the lower ends of links 22 of the lower set thereof . as best shown in fig6 however , there is secured to base plate 14 outwardly from the lower ends of links 22 of the lower set , a pair of guide channels 52 , which are horizontal and open faces thereof are disposed inwardly toward each other respectively for the reception of anti - friction guide members 54 . in operation , the guide members 54 and the anti - fraction rollers 40 simply move longitudinally within their respective guide channels 52 and 42 , respectively , at the lower and upper ends of the base unit as the lazy tong assemblies respectively are expanded and contracted simultaneously to raise and lower the upper supporting plate 10 . hence , it will be seen that rigid transverse brace members 44 , 46 , 48 and 50 provide stability for the lazy tong assemblies to resist swaying or skewing in one transverse direction , whereas the pivot bolts 36 and anti - friction followers 40 at the upper end of the lazy tong assemblies , together with the pivot bolts 32 and anti - friction guide or follower members 54 at the lower ends of said lazy tong assemblies provide resistance to swaying or skewing of the lazy tong assemblies in a lateral direction transverse to the direction against which said rigid bracing members resist swaying or skewing . the prime mover in the power mechanism by which the lazy tong assemblies 18 and 20 are expanded and contracted simultaneously comprises , preferably , an electric motor 56 of fractional horsepower rating , such as of the order of 1 / 8 horsepower , but such rating is not to be regarded as restrictive . however , because of the linkage mechanism , drive screw and other details of the power system which are described hereinafter , it will be seen that the base comprising the present invention for supporting , raising and lowering seat and back means connected thereto is very efficient and economical from the standpoint of power consumption . as shown in fig1 - 3 , motor 56 is enclosed within a suitable housing 58 to provide suitable aesethics for that portion of the base . also , the drive shaft of the motor 56 is connected to a gear reduction unit 60 which has an output drive shaft 62 . mounted on the outer end of drive shaft 62 is a drive sprocket 64 around which a sprocket chain 66 extends , the same also extending around a driven sprocket gear 68 , see fig6 which is fixed to one end of drive screw 70 . drive screw 70 preferably is of the acme type and extends through a transverse yoke 72 which has a central threaded bore complementary to screw 70 . it is to be noted from fig1 - 3 that the screw 70 is substantially horizontal and parallel to base plate 14 and , from fig6 it will be seen that said screw is substantially centrally between lazy tong assemblies 18 and 20 . opposite ends of the screw 70 are respectively rotatably supported within appropriate bearing blocks 74 and 76 . a pair of parallel connecting rods 78 are pivotally connected at one end to opposite ends of the transverse yoke 72 , as clearly shown in fig6 and the opposite ends are pivotally connected to one end 80 of a bell crank unit comprising a pair of similar bell crank members 82 , the shape of which is best shown in fig1 - 3 . the bell crank members 82 readily may be formed from metal plate stock and the intermediate portion of each of said members 82 has a boss 84 connected thereto , such as by welding . said intermediate portions and bosses 84 are bored respectively to receive the opposite ends of a transverse supporting shaft 86 . the opposite terminal ends of shaft 86 extend through complementary holes formed in the upper ends of a pair of similar vertical plates which jointly comprise a composite pivot member 88 which is rigidly connected to and supported by base plate 14 . the opposite or upper ends 90 of bell crank members 82 are connected by pivots 92 respectively to one end of each of a pair of connecting links 94 . the opposite ends of connecting links 94 are pivotally connected to pivot bolts 28 , which also interconnect the lower pair of crossed links 22 and 24 of each of the lazy tong assemblies 18 and 20 . the electric motor 56 is of the reversible type and is controlled by suitable switch means , not shown , which , for example , preferably are mounted upon the back of a chair seat when supported by the upper supporting plate 10 . however , the present invention also contemplates foot - operated switch members which are illustrated in fig6 and comprise pedals 96 . referring to fig7 it will be seen that a conduit 98 extends from a suitable source of current , not shown , and is positioned beneath the bottom plate 12 within suitable space therein provided for such conduit , said conduit leading to a terminal board 100 , shown in fig6 . said terminal board preferably is enclosed within the housing 58 and appropriate circuit control means , such as relays and the like , not shown , also are included within said terminal board 100 . a circuit also extends between the switches 102 , which are actuated by the foot pedals 98 and the motor 56 . any additional manually operable switch means , such as those referred to above as being on the back of a chair seat likewise are connected by appropriate circuitry , not shown , to the terminal board 100 . it is to be understood that all of such circuitry is relatively simple and of conventional nature and actually does not comprise part of the novelty of the present invention . for purposes of concealing the lazy tong assemblies 18 and 20 and the mechanism by which the same are actuated , the chair base comprising the present invention is provided with an accordian type flexible cover 102 , which is expansible and contractable vertically . the cover extends entirely around the circumference of the upper supporting plate 10 and the upper end of said cover is clamped to the periphery of said supporting plate by a flanged clamping member 104 , which surrounds said upper end of cover 102 and any suitable means such as screws or otherwise , not shown , are employed to clamp said upper end of cover 102 to the upper supporting plate 10 . similarly , a stiff shroud 106 , which may be formed from metal , molded from plastic or a rigid nature , or otherwise , surrounds the lower portion of the operating mechanism for the lazy tong assemblies 18 and 20 . said shroud , at the forward end of the base unit , interfits with housing 58 that encloses the electric motor 56 and otherwise is suitably secured to the base plate 14 so as to be movable therewith when the same is rotated about the pivot 16 with respect to the flat bottom plate 12 . the lower end of flexible cover 102 also is clamped to an inwardly extending flange 108 on the upper edge of shroud 106 by means of another flanged clamping member 110 which extends around the inner periphery of flange 108 and is secured thereto with the rim of the flexible cover 102 sandwiched therebetween by the use of screws , not shown , or any other suitable means . as described above , the lazy tong assemblies 18 and 20 , the upper supporting plate 10 and all of the operating mechanism for the lazy tong assemblies are mounted upon base plate 14 for support . said base plate and all of the mechanism mounted thereon is movable about the vertical pivot 16 with respect to the flat bottom plate 12 . such movement of the base plate 14 is facilitated by the provision of a plurality of anti - friction rollers 112 , which are positioned within suitable notches formed in the periphery of base plate 14 as best shown in fig6 and 7 . the rollers 112 operate upon a smooth area in the upper face of the flat bottom plate 12 . from fig1 it will be seen that the flat bottom plate 12 preferably is provided with a circular recess 114 in the upper surface thereof which receives the base plate 14 and the upper surface of the flat bottom plate 12 , which extends outwardly from the periphery of circular recess 114 , slopes downwardly around all edges of the plate 12 to prevent accidental tripping of an operator upon the outer periphery of said flat bottom plate 12 . after the base plate 14 has been disposed in a desired rotary position with respect to the flat bottom plate 12 , it is locked in said position by a relatively simple but effective locking means in the nature of a brake unit 116 . details of said brake unit 116 are best illustrated in fig6 - 9 , and the details thereof are as follows : a bridging plate 118 is secured by suitable screws 120 , see fig8 to a supporting block 122 , which is connected by additional screws 124 to base plate 14 . a shaft 126 extends through suitable bearings in the outer flange 128 and supporting block 122 , said shaft extending beyond both of said bearings . a foot treadle 131 is secured to the forward end of shaft 126 . the opposite end of shaft 126 is received within a suitable bearing hole in another supporting block 130 , which is fixed by welding within a suitable recess 132 formed within the base plate 14 . also supported within the recess 132 is a slide 134 which , on the inner end thereof has a cam surface 136 . the outer end of slide 134 has a brake block 137 of friction material fixed thereto for engagement with the wall of circulr recess 114 in the flat bottom plate 12 , as clearly shown in fig8 and 9 . slide 134 is guided for radial movement within recess 132 by a suitable channeled bracket 138 , the outer ears of which overlie the top surface of base plate 14 , as clearly shown in fig9 and are secured to said surface by appropriate screws . adjustably mounted upon the shaft 126 between the supporting blocks 122 and 130 is a cam actuator comprising a head 139 , see fig8 secured by a set screw to the shaft . the head 139 has a small shaft projecting therefrom which rotatably supports a roller 140 which , as best shown in fig9 engages cam surface 136 when the end 142 of foot treadle 131 is depressed . following the initial engagement of the end 142 of foot treadle 131 by the foot of the operator , continued movement of said end 142 will cause the roller 140 to move from the full line position thereof shown in fig9 to the dotted line position , and such movement causes the brake block 137 to be moved into firm frictional engagement with the vertical wall of the circular recess 114 in flat bottom plate 12 , and thereby lock the base plate 14 against any further rotation with respect to the flat bottom plate 12 . when the roller 140 is in the dotted line position thereof shown in fig9 it abuts the flat transverse end 144 of slide 134 and when in this position , it will automatically be retained therein until the opposite end of the foot treadle 131 is depressed , whereby such movement will restore the roller 140 to the full line position thereof shown in fig9 and thereby unlatch the brake block 137 so as to free the base plate 14 for rotation , when desired . from fig8 it also will be seen that a screw 146 is threaded into supporting block 130 coaxially with the socket therein which receives one end of shaft 126 . the inner end of screw 146 abuts said socketed end of shaft 126 and thereby assures effective operation of the cam - actuating roller 140 , since the screw 146 prevents axial movement of the shaft 126 toward the right as viewed in fig8 . therefore , it will be seen that the chair base comprising the present invention includes a simple but effective mechanism for locking and unlocking the base plate 14 and the mechanism supported thereby against rotation relative to the recess 114 within which base plate 14 is positioned relative to flat bottom plate 12 . from the foregoing , it will be seen that the present invention provides a chair base which is capable of elevating and lowering an operatory chair seat and back , such as those commonly used in detail operatories and other similar uses in the medical profession such as ophthalmology and other areas of the medical profession . the lazy tong mechanism is of such nature that it permits the seat of a chair to be extended to a very low position and the compact arrangement of the operating mechanism for the lazy tong systems also has been designed to achieve this advantageous result . the pair of lazy tong systems which are disposed in transversely spaced relationship are adequately braced to prevent swaying or canting of the seat and back when supported by said base . further , the elevation of the seat from its lowest position occurs smoothly and gently due to the particular linkage and bell crank arrangment which actuates the lazy tong system . further , substantial mechanical advantage is provided to effect such raising and lowering of a chair seat by said base , whereby a relatively low - powered electric motor is fully capable of effecting such raising and lowering of a chair seat and back by said base . further , all of the lazy tong systems and the power mechanism to expand and contract the same is enclosed within attractive expansible cover means and all of said mechanism is rotatable upon a bottom plate which rests upon a supporting surface , such as a floor and , following a rotation of the chair base unit relative to said bottom plate , effective brake means are provided to secure said rotatable base unit in a desired position upon said bottom plate . while the invention has been described and illustrated in its several preferred embodiments , it should be understood that the invention is not to be limited to the precise details herein illustrated and described since the same may be carried out in other ways falling within the scope of the invention as illustrated and described .