47 z Major 'General Roy's Account of the 



pears to coincide with the dart on the plane part of the brafi^. 

 is that which ferves by its motion to regifter the turns of the 

 head, h forked key, fitted to enter the holes near the circuiTi- 

 ference of the circle, is- made ufe of for the adjuflment of this, 

 zero* The circle (honld never be turned backwards or towards 

 the left, left the watch chain fliould thereby be thrown off tiie 

 barrel, but always forwards or towards the right, even if it 

 iliould be neceflkry to move it almoft an entire revolution. The 

 zero of the head is that which fliould be fii-fl: brought to cor-- 

 jefpond with its proper dart. They may be feen to coincidje 

 in the horizontal fedion of the micrometer; and the departure 

 of zero from this dart, indicates, by the number of divifions 

 that are intercepted, the value of any fradional part of a: 

 revolution. 



Account of the experiments with the Pyrometer^. 



Although the inftrument which I have here endeavoured to 

 defcribe was begun early in the winter of 1784, yet it was not 

 iiniflied till the beginning- of laft April ; at which time it was 

 brought to Argyll-Street, and being placed truly level on the 

 ftone pavement of the yard, was covered with an oil-cloth 

 canopy, that the experiments might not be interrupted by 

 rainy weather. 



To fill the three troughs completely it required from twenty- 

 five to thirty pounds of pounded ice, which was always put in with 

 great care, fo as to apply as compact ly as poffible to the ftan- 

 dard prifms and rod refpedively, with but little common water * 



* When common water was ufed, although not in any very confiderable pro- 

 portion, the therrhoa>eter kept always half, and fometimes three quarters of a 

 ilegree above 3a", 



at 



