658 Col. roy’s Experiments for 
to be arithmetical, or uniformly the fame in all parts of 
the fcale, will be .00312461 (a> . 
Having now fhewn the expanfion of quickfilver in the 
tubes of barometers refulting from the Geneva obferva- 
tions, I fhall next proceed to give fome account of thofe 
I made for that purpofe. They derive their origin from 
my having very accidentally obferved, that a fmall de- 
gree of heat, and of fhort duration, fenfibly affedted the 
length of the column in Mr. rams den’s portable baro- 
meter, whereof a view is given in plate XVI. The 
principal parts of this inftrument are a fimple ftraight 
tube, fixed into a wooden ciftern, which, for the conve- 
niency of carrying, is fhut with an ivory fcrew, and, that 
being removed, is open when in ufe. Fronting this aper- 
ture is diftindtly feen, the coincidence of the gage-mark, 
with a line on the rod of an ivory float, fwimming on the 
furface of the quickfilver, which is raifed or deprefled by 
a brafs fcrew at the bottom of the ciftern. From this, as 
a fixed point, the height of the column is readily mea- 
fured on the fcale attached to the frame, always to 
(a) This paper having lately been communicated to Mr. de luc, he hath 
informed me, that the difference of temperature in his -experiments, amounted 
to about 3 1 ° of reaumur, or 72 0 of Fahrenheit,, above freezing : where- 
fore, .00312461x72=1.225 nearly, will denote the rate of expanfion from 
which he deduced that for ’8o°; and within thefe limits, it will hereafter be 
ound to differ very little from the refult of the prefent experiments. 
T 
part 
