474 Major-General Roy's Account of the 



Two obfervers are neceflary for the effeilual application of the* 

 pyrometer. He whooblerves with the fixed mlcrofcope, takes 

 care that its object lens is kept in its true place,, that is to fay,. 

 that the wire in the eje-piece accurately biiedts the interfetliou. ' 

 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 hj obferving the plan (at WX) along with the ele- 

 vation of that end placed near it. The apparatus confifts of 

 two milled- headed fcrcws, working in bralB plates faftened to 

 the end of the frame, and acliijg againil; a fmall cock whicb 

 projedls from the lower part of the boiler, whereby this lafl 

 receives fuch longitudinal motion to and fro on its rollers, as^ 

 Is fufficient for the adjuftment of the lens. He who obferves^ 

 with the micrometer microfcope, having brought the zero of 

 the micrometer head to its dart, as fliewn in the horizontal^ ' 

 fedion, and alfo 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 bife£ls the in- 

 terfe£lion wires of its proper mark. This he effeds by work- 

 ing with the milled-headed fcrew, reprefented in the plan and- i 

 elevation of that mark, whereby the mark itfelf is moved 

 until the bifedlion is accurate ; and during the vi^hole of this 

 time, the firft obferver muft be extremely attentive to k^ep his 

 lens adjufted. 



One affiftant at leafl is neceffary, who takes his ilation on 

 the oppofite fide of the pyrometer, to obferve the levels, and 

 keep them adjufted, by means of the double hook applied^ near 

 the middle of the boiler, and reprefented in the fedion on the 

 line TU, at the lowermoft right-hand angle of the plate. 



The pyrometer having been adjufted in the manner here de- 



fcribed, by giving fufficient time for the flandard prlfms and 



I ' rod 



