• 47 2 Major-General Roy’s Account of the 
pears to coincide with the dart on the plane part of the brafs, 
is that which ferves by its motion to regifter the turns of the 
head. A forked key, fitted to enter the boles near the circum- 
ference of the circle, is made ufe of for the adjuftment of this 
zero. The circle fhould never be turned backwards or towards 
the left, left the watch chain fhould thereby be thrown off the 
barrel, but always forwards or towards the right, even if it 
fhould be neceftary to move it almoft an entire revolution. The 
zero of the head is that which fhould be firft brought to cor- 
refpond with its proper dart. They may be feen to coincide 
in the horizontal feCtion of the micrometer ; and the departure 
of zero from this "dart, indicates, by the number of divifions 
that are intercepted,, the value of any fractional part of & 
revolution. 
Account of the experiments with the Pyrometer .. 
Although the inftrument which I have here endeavoured to 
deftcribe was begun early in the winter of 1784, yet it was not 
finifhed 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 wfith 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 compaClly as poffible to the ftan- 
dard priftns and rod refpeClively, with but little common water * 
* When common water was ufed, although not in any very confiderable pro- 
portion, the thermometer kept always half, and fometimes three quarters of a 
degree above 3 2°. 
at 
