46.2 Major-General Roy^s Account of 'the 



For, fuppofmg both fets of experiments to have been made with 

 iuflruments equally perfe<£l, and to have been in other refpe^s 

 equally well conducted, this muft always be coniidered as the 

 mod: unexceptionable method. Bciides, the expaniion of rods 

 of the length of five feet being nfcertained, the unavoidable 

 error of obfervations of this delicate nature, becomes leffened 

 In proportion to the excels of their length above (liorter rods. 

 'In thefe new experiments too, another fort of pyrometer, in- 

 vented by Mr. Ramsden, has been applied, of fuch accurate 

 conftruclion that it feems not eafy to improve it. 



The microfcopic pyrometer, lo named becaufe, by means 

 ^of two microfcopes attached to it, the expanfion is meafured, 

 confifts of a ftrong deal frame five feet in length, nearly 

 twenty-eight inches broad, and about forty- two inches in 

 height. The elevation of the eye-piece fide, or that which 

 prefents itfelf to the obferver, and alfo of the micrometer end, 

 -or that which is towards his right-hand, as well as the general 

 .plan of the top, are reprefented by a fcale of one inch to a 

 foot, or one-twelfth part of the real dimenlions, in tab. XX. 

 •^vhere likewife may be {t^w the angular view of the fixed end, 

 together with plans, fedlions, and elevations, of feveral of the 

 principal parts, done to larger fcales. From thefe, it is hoped, 

 ^he conftrudion of the machine will be eafily underftood, 

 without entering into a minute defcription of the almofl num- 

 •^berlefs fmaller parts whereof it is compofed. 



On the top of the frame, two deal troughs, upwards of five 

 ■feet in length, are firmly fcrewed. That towards the obferver 

 overhangs the frame fomething more than an inch : that on 

 ■the farther fide is even with the back part. Each of thefe 

 ^troughs, which, are about three inches fquare in the infide, 

 icontains a caft-iron llandard prifm, whofe fides are i \ inch. 



The 



