a counter - balance weight ( 20 ) comprises a substantially solid body member of grey cast iron . the weight is of a rectangular shape as viewed in plan , as shown in fig4 . the body member is provided with a through - bore ( 21 ) which extends between transverse end faces ( 22 , 23 ), each said end face being of a substantially triangular shape . the body member ( 20 ) is of a substantially symmetrical construction such that said triangular shape is that of an isosceles triangle , and the through - bore ( 21 ) is positioned mid - way between edge regions ( 24 , 25 ). the body member thus has a major face ( 26 ) which in use is intended to rest on a support surface such as that provided by a ground surface or roof surface , and a pair of shorter , relatively inclined upper surfaces ( 27 , 28 ) which define therebetween an apex region ( 29 ). a boss ( 30 ) and screw threaded aperture ( 31 ) are provided mid - way along the length of the apex region , the boss protruding beyond said triangular shape . aperture ( 31 ) extends in a direction perpendicular to said major face ( 26 ). the volume of material of the body member in this embodiment is 40 % of the volume of the smallest cuboid that can contain the body member , i . e . that cuboid having a volume equal to the height , width and length of the body as viewed in fig3 and 4 . in this embodiment of the invention the apex angle between the upper surfaces ( 27 , 28 ) is 150 °, the angle between each upper surface and the major surface ( 26 ) therefore being 15 °. the transverse end faces ( 22 , 23 ) depart from a true triangular shape at the apex region , at which the body member is slightly curved . the end faces depart from a true isosceles triangle shape also at the edge regions ( 24 , 25 ) which are truncated to lie inwards from the point of intersection of the longer and shorter sides of the shape of the end faces ( 22 , 23 ), ie the intersection point ( p ) of the surfaces ( 26 , 27 ) as shown in fig3 . the extent to which said edge regions ( 24 , 25 ) lie inwards from the respective points of intersection is selected such that the height ( h ) of the edge regions , as considered in a direction perpendicular to the face ( 26 ), is more than the maximum height ( b ) by which the top surface ( 32 ) of the boss ( 30 ) extends above the apex region ( 29 ). in this embodiment the ratio of the width of the body member , being the length of the longer side ( 26 ) as viewed in fig3 , relative to the height , being the spacing between the surfaces ( 26 , 32 ), is 4 . 5 : 1 . the through - bore ( 21 ) of circular cross - sectional shape has a diameter which is in the order of two thirds of the height of the body ( 20 ) i . e . two thirds of the distance between the surfaces ( 26 , 32 ). the longitudinal axis of the through - bore is positioned to lie substantially mid - way between said surface ( 26 ) and the upper surface ( 32 ) of the boss . fig7 illustrates a plurality of weights arranged in a compact configuration for transport . alternate weights are inverted , and both the inverted and non - inverted weights are slightly spaced apart such that the boss region ( 30 ) of one weight can lie between the spaced apart truncated edge regions ( 24 , 25 ) of two other weights . a counter - balance weight ( 50 ) in accordance with another embodiment of the invention is of a substantially similar construction to that shown in fig2 to 6 , but differs in respect of the through - bore . the through - bore ( 35 ), see fig8 and 9 , is of a circular cross - sectional shape at a central position mid - way between the ends ( 36 ) and opens outwards , with increasing dimension in a direction perpendicular to the major face ( 37 ) of the weight , to be of an oval shape ( 38 ) at each end . in consequence when a spacer tube of an outer diameter close to the diameter at the mid - length position of the bore is inserted in the bore , the tube can be tilted slightly relative to the plane of the major face ( 37 ) but the uniform width of the bore ( as viewed in the transverse section of fig8 ) ensures that the counter - balance weight remains aligned with the length of the tube . one suitable type of base unit for use in combination with the aforedescribed counter - balance weights is shown in cross - section in fig1 and is of a kind the subject of our co - pending uk patent application gb 02 21009 . 4 . the base unit ( 41 ) comprises a cast iron body having a first socket ( 53 ) to receive the lower end of a vertical post and a horizontal socket ( 54 ) to receive the end of a spacer bar . each socket tapers slightly such that a post or spacer bar can be secured relative to the body of the base unit in any position within a prescribed angle range , in this case an angle range of plus or minus 5 degrees . fig1 shows a short length of a safety rail installation ( 40 ) of a modular , free - standing type comprising base units ( 41 ), vertical posts ( 48 ), horizontal top and intermediate safety rails ( 44 , 45 ), horizontal spacer bars ( 42 ) and counter - balance weights ( 50 ) secured to the distal end of each horizontal spacer bar ( 42 ). the counter - balance weights ( 50 ) are each of the type described above either with reference to fig2 to 6 , or fig8 and 9 . they are each secured to a spacer bar by means of a grub screw fitted within the screw threaded aperture of the boss ( 30 ) and tightened to bear firmly against an outer surface of the spacer tube . in this assembly the end spacer bars each have three weights ( 50 ) secured thereto , the weights being provided directly adjacent one another thereby to present substantially smooth and continuous upper inclined surfaces . the two intermediate spacer bars each have only one weight ( 50 ) secured thereto . fig1 a , 12 b and 12 c show a weight 60 having end faces 61 which are each planar but lie in a non - parallel manner , inclined inwards away from the major , base surface 62 , in this case at an angle in the range 55 ° to 80 °. that angle is greater than the angle by which the upper faces 63 lie inclined relative to the surface 62 . as best seen from fig1 c , when considered in a sideways direction the body has a non - rectangular profile . other features of the weight 60 are similar to those of the weight of fig2 , except that there is no protruding boss 30 . instead the screw - threaded aperture 64 is surrounded by a flat land 65 that extends outwards from the axis of the aperture by a distance equal to the diameter of the aperture . fig1 a , 13 b , 13 c show a weight 70 which is a variation of that of fig1 . the end faces 71 lie at a lower inclination , of 45 °, though even lower angles , down to that of the inclination of the upper faces 73 is envisaged . the weight 70 differs also in being provided with a protruding boss 72 . fig1 a , 14 b , 14 c show a weight 80 which has end faces 81 that differ from those of fig1 . the faces 81 each comprise in part a planar section 82 that extends perpendicularly from the base surface 83 , and a curved section 84 which leads , in an inwards direction , to the upper inclined surfaces 85 . fig1 a , 15 b , 15 c , 15 d show a weight 90 which is similar to that of fig1 but has an upper surface 91 of curved shape , being substantially part elliptical as viewed in fig1 d . each end face 92 comprises planar and curved portions 93 , 94 corresponding substantially with the portions 82 , 84 of the weight 80 of fig1 . fig1 a , 16 b , 16 c , 16 d show a weight 100 which is similar to that of fig1 , but has planar end face portions 101 which lie at a greater angle of inclination . that angle preferably is in the range 30 ° to 60 °, and is approximately 45 ° in the illustrated embodiment . the embodiments of fig2 to 9 and 12 to 16 each have footprints of a substantially rectangular shape . in the embodiment illustrated in fig1 a , 17 b , 17 c , 17 d a weight 110 has a footprint which is non rectangular , as best seen from 17 b . the two end faces 111 each extend perpendicularly relative to the base 112 , but are convexly curved , being curved in a substantially elliptical shape . other shape features are substantially similar to those of the weight of fig2 .