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1871.] 117 
(Heller and Brightiey. 
way than the short centre Transit, but unlike the short centre, keeps all 
the centres covered and not removable from the instrument, and leaves 
the tripod head and legs with the four levelling screws, ete., to be carried 
by his assistant. The difference in weight will be appreciated by the 
Railroad Engineer when we inform him that a plain Transit with all its 
centres, ete., only weighs about as much as a Surveyor’s Sight Compass, 
and is more steady and keepsin adjustment better than the ordinary long 
centre transit, weighing from 25 to 80 pounds. 
i28) ® 
The City Engineer has in this instrument all the advantages of the 
ordinary ‘long centre Transit’? with only half the weight, and an in- 
crease of steadinsss. 
There are several defects that are common to all Transits, among which 
are— 
1st. The “tangent or slow motion screw’’ that moves the upper or 
vernier plate, by use becomes worn and does not fit precisely the thread 
in the interior of the nut through which it passes. When this occurs the 
tangent screw can be turned sometimes a complete revolution without 
moving the vernier plate. This ‘lost motion”? or “back lash”’ of the 
tangent is one of the worst annoyances of Engineers, aud has been the 
source of serious errors in the field. Several methods have been devised 
to overcome this which we will here describe. The nut through which 
the screw works has been made in two sections to allow of being drawn 
together when the screw wears. This plan would answer ifthe screw always 
wore equally in every portion of its length, in other words was a cylinder, 
but this it never does, and if the nut is tightened so that the lost motion 
is removed from the thinner portion of the screw, it will move so tightly 
as to be useless when it comes to the portions that are not worn so thin. 
There are several methods of drawing the nut together, but they have 
all the same objections as the above, that is, they are not effective in the 
entire length, and the nut must be pressed so very hard on the screw as: 
to make the working of the tangent very tense, especially in cold weather. 
Another, and the last method has been to apply a long spiral spring be- 
tween the nut and the head of the screw that acts as the finger piece, 
thus pressing the nut and the screw from each other, and consequently 
removing all ‘lost motion’? from the screw. This plan though in 
theory very good, in practice has been found inoperative, for the 
following reason: the spiral spring had of necessity to be made long 
enough, and stiff enough, to act in every portion of the screw’s length, 
the alternate opening and closing of the spring by use weakened it, and 
in a short time it failed to remove the ‘‘ back play.’”’ To get rid of this 
defect of ‘‘lost motion’’ in the tangent screw, opposing or butting screws 
have been sometimes substituted, but in use they do not give satisfaction, 
as two hands must be employed in using them, and standing from the 
edge of the plate, they are liable to be injured by blows, and they are 
apt, unless very carefully used, to throw the instrument out of level. 
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