-690 Col. roy’s Experiments for 
whofe bores were commonly from T '-th to -Tth of an 
inch in diameter. The capacity of the tube was care- 
fully meafured, by making a column of quickfilver, about 
three or four inches in length, move along it from one 
end to the other. Thefe fpaces were feverally marked, 
with a fine edged file, on the tubes ; and transferred from 
them to long flips of paftc-board, for the fubfequent 
conftrudlion of the fcales refpedlively belonging to each. 
* 
The bulb, attached to one end of the manometer at the 
glafs houfe, was of the form of a pear, whofe point being 
occafionally opened, dry or moifl: air could be readily ad- 
mitted, and the bulb fealed again, without any fenfible 
alteration in its capacity. 
The air was confined by means of a column of quick- 
filver, long or fhort, and with the bulb downwards or 
upwards, according to the nature of the propofed expe- 
riment. Here it muft be obferved that, from the adhe- 
fion of the quickfilver to the tube, the inftrument will 
not a<5t truly, except it be in a vertical pofition; and even 
then - , it is neceflary to give it a fmall degree of motion, 
to bring the quickfilver into its true place ; where it will 
remain in equilibrio, between the exterior preflure of 
the atmofphere on one fide, and the interior elaftic force 
of the confined air on the other. 
All 
