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XIX. Defer tption of a Jimple Micrometer for meafuring frail 
Angles with the ‘Telejcope. By Mr . Tiberius Cavallo, 
F. R. S, 
Read June 2, 179 1. 
T HE various telefcopical micrometers, or machines which 
have been conftruded for the meafurement of fmall 
angles, may be divided into two claffes ; namely, thofe which 
have not, and thofe which have, fome movement amongft 
their parts. The micrometers of the former fort confift moftly 
of fine wires, or hairs, varioufly difpofed, and fituated within 
the telefcope, juft where the image of the objed is formed. 
In order to determine an angle with thofe micrometers, a good 
deal of calculation is generally required. The micrometers of 
the other fort, of which there is a great variety ; fotoe being 
made with moveable parallel wires, others with prifms, others 
again , with a combination of ienfes, and fo on ; are more or 
lefs fubjed to feveral inconveniences, the principal of which 
are the following. ift, Their motions generally depend 
upon the adion of a ferew, and of courfe the imper- 
fedions of its threads, and the greater or lefs quantity of loft 
motion, which is obfervable in moving a ferew, efpecially 
when fmall, occafion a confiderable error in the menfu ration of 
angles. 2dly, Their complication and bulk renders them dif- 
ficultly applicable to a variety of telefcopes, efpecially to the 
Pp 2 pocket 
