RECENT PROGRESS IN PHYSICS. 435 



wliicK the metallic cone g with a split bead is screwed, as is more 

 clearly shown in fig. 55. In the slit the platinum wire is held and 

 is firmly clamped by screwing the cone in deeper. Riess terms this 

 contrivance a "cone clamp." 



When one wire is to be taken out in order to introduce another one, 

 the method pursued is as follows: The cap h is first unscrewed; 

 the cylinder /, of one of the sides, is lengthened externally by a 

 screw, to which another of less diameter is attached. On this last, 

 as shown in Fig. 55, is fastened a rod, whose length is greater than 

 the diameter of the globe together with the metallic attachments ; 

 when this is done, the metallic plate ic, which prevents the cylinder 

 /from being drawn into the globe, can be detached and slipped on 

 the rod. It is now easy to draw the cylinder/from the left side of 

 the globe, and push the rod, with its attached cylinder, on the other 

 side after it ; the wire may then be removed and another put between 

 the cone clamps. To replace the cylinder, the rod is first passed 

 through the globe drawing / with it, the plate x is screwed into its 

 place, and the rod being removed, the two caps h are again screwed 

 on. 



The air thermometers, constructed by Kleiner, are very beautiful 

 and well made, but their price (25 thalers and 2 thalers for packing) 

 is high. It is greatly to be desired, on this account, that the instru- 

 ment should have a simpler construction, which would render it less 

 costly. 



The inclination of the tube, as seen in Fig. 53, can be changed at 

 pleasure, and then the sensibility of the instrument increased to any 

 desired degree. Biess used generally an inclination of 6^ degrees to 

 the horizon. 



The scale of the tube is divided into lines. All the other parts are 

 clearly shown in the figure. 



The capacity of the globe of Riess' instrument was 40,766 cubic 

 lines. The size of the tube was such that, the space between two 

 -division lines being taken as unity, the globe contained 320,307 sucb 

 units of capacity. 



§43. Theory of the instrument. — When the air in the globe is 

 heated, the column of liquid is depressed; thus, on the one hand, the 

 tension of the air, and, on the other, its volume, is increased. 



But the increase of tension, as well as that of volume, is propor- 

 tional to the rise of temperature ; hence the sum of both effects, or 

 the depression, is proportional to the rise of temperature. 



We will now coujpute the increase of the temperature of the air in 

 the globe, which produces a depression of one line. 



Suppose the temperature to be 15° Cent, and the barometer to indi- 

 cate 336 lines. 



The liquid in the tube was 15 times lighter than mercury; hence 

 a barometer of this liquid would have a height of 15 X 336 = 5040 

 lines. 



But the tube is not in a vertical position, it is 6°. 5 to the horizon. 

 The column of liquid in a tube thus inclined must have the length 



5040 5040 ., ... T 



