RUDBERG ON THE EXPANSION OF BRY AIR. 



509 



at most was requisite, with even the smallest of the tubes which 

 I employed. 



Snow was now placed on 

 the metal dish H, and the 

 globe surrounded with it 

 on all sides. The water 

 produced by the melting of 

 the snow escaped through 

 the tube L. As soon as 

 the snow began to melt, 

 fresh snow was carefully 

 added, so that the tempe- 

 rature of the globe was 

 kept at 0° for about two 

 hours, and sometimes even 

 longer. "When by this 

 means I was certain that 

 all the mercury had ac- 

 tually entered which at 0° 

 could be forced in by the 

 pressure of the atmosphere, 

 I closed the fine opening 

 of the tube with a very soft mixture of wax and turpentine, 

 which was prepared for that purpose in a little spoon of iron. 

 At the same instant the barometer was observed, in order to de- 

 termine the existing pressure of the atmosphere ; the snow was 

 then carefully removed, and the difference of altitude between the 

 surfaces of the mercury within and without the globe measured. 



For this purpose the measuring apparatus N M K was pre- 

 pared. Upon the whole, it depends upon the principle employed 

 in measuring the height of the mercury in Fortin's barometers. 

 A slider M, embracing tightly the vertical bar, is moved up or 

 down by a screw P, and therefore also the cylindrical ring N, 

 and screw K, which are connected with it. The ring N having 

 first been made accurately horizontal, was depressed, surround- 

 ing the globe, till its under edge coincided with the surface of 

 the mercury in the globe, and the screw K S turned till its point 

 S just touched the surface of the mercury in the trough. It is 

 evident that the difference of altitude between the under edge 

 of the ring and the point S was equal to the difference of alti- 

 tude of the two surfaces of the mercury. After the contacts hud 



