462 Major-General Roy’s Account of the 
For, fuppofing both fets of experiments to have been made with 
inftruments equally perfedl, and to have been in other refpedls 
•equally well conducted, this muft always be confidered as the 
moft unexceptionable method. Befides, the expanfion of rods 
mf the length of five feet being afcertained, the unavoidable 
terror of obfervations of this delicate nature, becomes leflened 
in proportion to the ! excefs of their length above fhorter rods. 
In thefe new experiments too, another fort of pyrometer, in- 
vented by Mr. Ramsden, has been applied, of fuch accurate 
•conftruction that it leerns not eafy to improve it. 
The •microfeepie pyrometer, fo named becaufe, by means 
of two microfcopes attached to it, the expanfion is meafured, 
confifis of a flrong 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 dimenfions, in tab. XX. 
where likewife may be feen 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, 
the conftrudtion of the machine will be eafily underftood, 
without entering into a minute defcription of the almofi: 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, 
contains a caft-iron ftandard prifm, whofe fides are 1 \ inch. 
The 
