70 A COMPARISON BETWEEN THE 



level of the mercury in the cistern ; no neutral point is marked 

 upon them, nor is the diameter of the bore of the tube ascertained ; 

 and in some the capacity of the cisterns is perpetually changing 

 from the stretching of a leathern bag. 



We have endeavoured, as far as possible, to obviate these diffi- 

 culties in the following manner — 



1st. Procure one or two of the best barometer tubes, and a suffici- 

 ency of clean mercury ; take care that the interior of the tube is 

 quite clean and dry ; heat it gradually before the fire ; — strain the 

 mercury through a fine clean and dry silk handkerchief, and then 

 heat it quite hot, or what is better, just allow it to boil ; when 

 cool enough, strain it again through the silk handkerchief into a 

 dry warm 2oz. glass apothecary's measure ; 2ndly, then Jill the 

 warm dry tube with it; whatever air-bubbles are seen, may be 

 easily removed by passing the largest up and down the tube. — 

 Again, 3rdly,j^/Z the tube quite full, and placing the Jtnger firmly 

 over the bore, plunge it into a cup of clean warm mercury ; on re- 

 moving the finger, the mercury falls a certain distance from the 

 top. Before taking the tube out of the mercury, place the end of 

 the finger again firmly over the bore, and then remove it. The 

 space above the mercury is now occupied by very rarefied air, and 

 by passing this three or four times very carefully up and down, 

 keeping the finger close on the open end, all the air sticking in any 

 part will be removed. Then allow the mercury (the finger being 

 still upon the open end of the tube) to run slowly down to the 

 closed end, taking care that no air-specks are any where visible ; 

 then remove the finger, fill the tube quite full, and repeat the 

 operation twice or thrice. Lastly, carefully fill the tube, and re- 

 move by a very slender piece of whalebone or strip of quill every 

 speck of air that appears at the junction of the two portions of mer- 

 cury, or elsewhere, above it. Again fill the tube quite full, and 

 seal it securely with sealing-wax — placing a piece of gummed paper 

 over the sealing-wax. 



Now take a quire of long, smooth, clean, white paper, that has 

 been exposed some little time to the air, — ^in order that it may not 

 contract by drying — and, drawing a pencil line along it, measure, 

 very carefully, 14 inches, which, by a pair of compasses, may be 



