68 ^ Sir g. shuckburgh’s Letter to Col. roy on 
the neceffity of correcting his rules. But although in 
this effential and fundamental part of the inquiry we 
agree, there are, neverthelefs, fome little circumftances 
wherein we differ; it is the fubjeCt of this letter, sir, to 
point out to you the degree of our differences, a compa- 
rifon that I had the pleafure flightly to exhibit to you a 
few days ago, and which I truft will not be found unim- 
portant to thofe who may be engaged in thefe purfuits: 
if, therefore, you judge thefe remarks of fufficient mo- 
ment, I will beg the favour of you to lay them before the 
Royal Society, as the belt means of communicating them 
to the public. f 
The two chief caufes of our difference are, the expan- 
lion of quickfilver and the expanfion of air. I fhall be- 
gin with the equation for quickfilver. 
The mean' temperature of ordinary barometrical ob- 
fervations, I apprehend, will generally be found to lie 
between 40® and 70° on Fahrenheit^ thermometer; 
now the mean expanfion in this range, according to your 
obfervation, is ,0323 inch on a column of 30 inches for 
io° of heat; by my table it is only ,0304 inch, the dif- 
ference ,0019 inch is equal to about 20 inches in the 
refult of the height, when the temperature of the two 
barometers differs io°, and this may reafbnably be ex- 
pected only in a height of 3000 or 4000 'feet. In an 
obfer- 
