112 ON DIVIDING ASTRONOMICAL INSTRUMENTS. 



uniform tern- taken duniig- the turning of the outer edge, to have the cir- 

 mrnmAlie ' ^^^ ^^ ^''^ same temperature; fov one part in<y be expanded 

 •uter edge, by heat, or contracted by cold, so much more than another, 

 as to cause the numbers in the tables of errpurs to be in- 

 conveniently laro-e. A night is not more than sufficient for 

 allowihij the whole to take the same temperature, after hav- 

 ing been handled by the workmen; and tlie tinishing touch 

 should be given within a short space of time. But, if the 

 effects of temperature are to be regarded in t\irning a cir- 

 and above all cle, it is of tenfold more importance to attend to this cir- 



vhLle examin- cumstance, while the examiiiation of the lar;4er arcs of the 

 TOg the larger _ o 



arcs. instrument is carried on; for it is absolutely necessary, that. 



Effects of tern- j^y,.;^ this time, the whole circle should be of the same 



perature much ^ _, 



nioreimportant heat exactly. Few workmen are sufficiently aware of this : 

 than geiierally 7'liey generally suppose the expansion of metals to be a tri- 

 fle, 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 Fahrenheit's 

 thermometer indicates so small a portion of heat, that, in 

 such places as workmen are usually obliged to do their bu- 

 siness in, it is not very easy to have three thermometers at- 

 tached to different parts of a large instrument, showing an 

 equality of temperature within that quantity: Y"et 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 semicircle will actually exceed the 

 lower by 2": And, if such should happen to be the case 

 vihWe the examination of the first dot of the third quadrant 

 is miide,the regularity. of the whole operation would thereby 

 be destroyed. 

 The apparatus ^t may not be improper to remark, that dividing by the 

 not expeiisive gye does not requ re a more expensive a{)paratus than the 

 operation of dividing by hand ; and, indeed, less so when 

 the scale of inches is deemed necessary. The method by 

 adiustment is still more expensive, requiring whatever tools- 

 Bird's method requires, and, in addition to these, a frame 

 and microscopes, somewhat similar to those for dividing by 

 the eye. 

 Much tii>:e It is somewhat more difficult to give a comparative esti- 

 mate 



