The 3- Screw Hase 



It would be difficult to say where ti base originated. 



It would be a safe speculation to say that it was the first slow 

 motion applied to the ball-and-socket joint with compasses, astro- 

 labes and planispheres in use in Europe prior to 1700, because the 

 ball-and-socket cannot be leveled in a rigid mounting by three 

 screws and only two hands with which to work. 



We shall probably never depart from the 4-screw base so long 

 as the ball joint is retained. Many of the more conservative adhere 

 to it by reason of its so called rigidity secured mainly by high 

 tension in the screws which, in turn, impart an unequal strain on each 

 of the four arms. This is especially evident in wye levels in which 

 the bubble will move off center with no apparent cause. It is a 

 practical impossibility to equalize the strain in each of the arms of 

 the 4-screw head. A slight change in temperature accentuates 

 these conditions and causes a binding known in England as "head- 

 ache." 



The 4-screw base is never used on any other type of scientific 

 apparatus. The 3-screw base is an ideal construction because the 

 centers are never distorted or strained by lateral pressure or jambing. 

 The longer arms permit better leverage and more sensitive control 

 and with numb fingers, in cold weather, the improvement is very 

 pronounced. The Germans and French have used the general 

 design shown in Fig. 73 which is, no doubt, best suited to the 

 heavier instruments that will not respond to slight easements in the 

 spring. The English overcame the necessity for a spring in their 

 "Tribrach" in which each leveling screw terminates in a spherical 

 base that provides for a universal seating as well as a means of lock- 

 ing the instrument to the lower plate. 



The special tripod heads necessary with all these models have 

 made the introduction of the 3-screw principle a \er\ deliberate 

 matter until we introduced the patented model shown in Figs. s 

 and 77 when a new impetus was created in its favor. This model 

 not only fits the ordinary tripod but is provided with the usual 

 shifting center and a knurled ring to lock it against easement while 

 carrying it on the shoulder. When setting up, the knurled ring 



ft* 



