CHAP. I.] THEODOLITE 19 



is very useful, and may prevent loss of a day's work, and much 

 aggravation to all concerned. 



The parts of a theodolite, especially in an old instrument, 

 that most frequently get out of order, are the small screws 

 which hold the milled heads of the tangent screws in their 

 places. A young observer is often much bothered and puzzled 

 by his instrument not coming back to zero, which may result 

 from many things, but most frequently from one of the small 

 screws above mentioned being loose. Screwing it up tight 

 enough to prevent any play when the instrument is clamped, 

 but not so tight as to make the tangent screw work hard, will 

 often remove the difficulty. 



Other causes of not coming back to zero are : — 

 1 . Looseness of the sockets througli which the tangent screws 

 work, and which can be easily tightened by their screws. 

 --2. Looseness of the fittings of the brass stand on the theo- 

 dolite legs There are many Avorking parts here, and any of 

 them are liable to get loose. The leverage on the brass plate 

 that fits on each leg head is enormous, if the leg should be 

 allowed to swing out in taking off the rings ; and as the screws 

 that hold them on are small, looseness may easily take place 

 here. 



3. In an old instrument, the faces of the clamping plates 

 may screw close together without clamping the instrument 

 tightly. This is from the part that holds the instrument, and 

 which gets all the friction, being much worn. The parts into 

 which the clamping screw fits must be smoothly filed on their 

 inner faces, so as to ensure the other parts coming into contact 

 with the body of the instrument, before the faces of the clamp- 

 ing plates meet. 



4. The upper plate will sometimes not revolve freely, but 

 catches every now and then. This is from the piece of metal 

 which clamps the two plates together either not fitting very 

 well, or being dirty inside, or perhaps bent. Placing the finger 

 underneath so as to press it up to the lower plate, whenever 

 the plates are to be revolved, will ensure its working smoothly, 

 and is a better thing for a young hand to do than to attempt 

 to take it off. The same thing will happen to the reading- 

 glass plate ; but here it is often the little screw underneath 

 which is loose, and simply screwing it up will relieve it. 



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