474 " Major-General Roy*s Account of the 
Two observers are neceffary for the effectual application of the 
pyrometer. He who obfervcs with the fixed microfcope, tak.es> 
care that its objcdt lens is kept in its true place, that is to lay, 
that the wire in the eye-piece accurately bifects the interfeCiioin 
wires of the mark.. This he is enabled to do by means of the 
apparatus attached to the fixed end of the boiler, as will be bed 
conceived by obferving the plan (at WX) along with the ele- 
vation of that end placed near it. The apparatus con fids of 
two milled -headed 1 crews, working in brafs plates fadened to 
the end of the frame, and acting againd a fmall cock which: 
projects from the lower part of the boiler, whereby this lad 
receives fuch longitudinal motion to and fro on, its rollers, as 
is fufficient for the adjustment of the lens. He who obfcrves 
with the micrometer microfcope, having, brought the zero of 
the micrometer head to its dart, as fhewn in the horizontal 
fedlion, and ald> the revolution zero to its dart, as reprefented. 
in the elevation, takes care,, when the rod has acquired the 
freezing temperature, that the micrometer wire bifedts the in- 
terfedlion wires of its proper mark. This he effeds by work- 
ing with the milled-headed fcrew, reprefented in the plan and 
elevation of that mark, whereby the mark, itfelf is moved 
until the bifedtion is accurate ; and during the whole of this 
time, the f rd obferver mud be extremely attentive to keep his 
lens adjuded. 
One afiidant at lead is neceflary, who takes his dation on 
the oppofte fide of the pyrometer, to obferve the levels, and 
keep them adjuded, by means of the double hook applied near 
the middle of the boiler, and reprefented in the fedtion on the 
line TU, at the lowermod right-hand angle of the plate. 
The pyrometer having been adjuded in the manner here de- 
ferred, by giving fufficient time for the dandard prifms and 
rod 
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