1 74 On dividing Instruments. 



on the result of each respective, examination, as here pre- 

 stntefi, there could have been more than one opinion, it 

 vouid nnt have appeared here. I am fariher prompted to 

 add, that the above comparative view pn-^ems one circum- 

 siance to owr notice, which cannot do less than eratify 

 every individaal who is at all conversant in there matttrs ; 

 I mean, the high ra'nk which general Roy's scale takes in 

 the li>t ; tliU scale havjng been made the agent in mea- 

 suring the base hue ot our national trigonometrical survey. 



To reuirii, finally, to the dividing of circles; I must 

 state, as matter of precaution, that great care should be 

 taken during the tunnng of the outer edtje, to have the 

 circle of the same temperature ; for one part may be ex- 

 panded bv heat, or conlractctl by cold, so much more than 

 another, as to cause the numbers in the tables of errors to 

 be inconvemcntly large. A night is not more tiian suffi- 

 cient for allowing tlie whole to take the same temperature, 

 after having been handled by the workmen; and the finish- 

 ing touch should i)c given within a short space of time. 

 But, if the eflccts of temperature are to be regarded iu 

 turning a circle, it is of tenfold more importance to attend 

 to this circumstance, while the examination of the larger 

 arcs of the instrument is carried on ; for it is absolutely 

 necessary that, during this time, the whole circle should 

 be v)f the same heat exactly. Few workiricn arc sufficiently 

 aware of this : they generally suppose the expansion of 

 metals to be a trifle which need not be regarded in practice ; 

 and wonder how the parts of a circle can be diHerently 

 heated wiihout taking pains to make it so. One degree of 

 Fahrenheit's thernrometer indicates so small a portion of 

 heal that, in sucli places as workmen are usually obligeri 

 to do their business in, it is not very easy to haver three 

 thermometers attached to different parts of a large instru- 

 ment, showing an equality of temperature within that 

 quantity : yet so necessary is correctness in this respect, 

 that if a circle has the vertex one degree warmer than i'ts 

 opposite, and if this ditlcrence of temperature be regularly 

 distributed from top to bottom, the uper semi-circle will 

 actually exceed the lower by 2": and, if such should hap- 

 pen to be the case while the examination of the first dot of 

 the third quadrant is made, the regularity of the whole 

 operation would thereby be destroyed. 



It may not be improper to remark, that dividing by the 



eve does not require a more expensive apparatus than the 



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



the scale of inches is dcetned necessary. The method by 



8 adjustment 



