Mr. Troughton on dividing Instruments . 3-4 r 
To return, finally, to the dividing of circles ; I must state, 
as matter of precaution, that great care should be taken during 
the turning of the outer edge, to have the circle of the same 
temperature ; for one part may be expanded by heat, or con- 
tracted by cold, so much more than another, as to cause the 
numbers in the tables of errors to be inconveniently large. A 
night is not more than sufficient for allowing the whole to 
take the same temperature, after having been handled by the 
workmen ; and the finishing touch should be given within a 
short space of time. But, if the effects of temperature are to 
be regarded in turning a circle, it is of tenfold more import- 
ance to attend to this circumstance, while the examination of 
the larger arcs of the instrument is carried on ; for it is abso- 
lutely necessary that, during this time, the whole circle should 
be of the same heat exactly. Few workmen are sufficiently 
aware of this : They generally suppose the expansion of me- 
tals to be a trifle which need not be regarded in practice ; and 
wonder how the parts of a circle can be differently heated 
without taking pains to make it so. One degree of Fahren- 
heit's thermometer indicates so small a portion of heat that, 
in such places as workmen are usually obliged to do their 
business in, it is not very easy to have three thermometers 
attached to different parts of a large instrument, shewing an 
equality of temperature within that quantity : Yet so neces- 
sary is correctness in this respect, that if a circle has the 
vertex one degree warmer than its opposite, and if this dif- 
ference of temperature be regularly distributed from, top to 
bottom, the upper semi-circle will actually exceed the lower 
by 2": And, if such should happen to be the case while the 
examination of the first dot of the third quadrant is made, 
