250 METEOEOLOGY AND ALLIED SUBJECTS. 



ervoirs are quite near together and exposed to the radiation of the sun 

 and sky. The counterpoise is so attached to the brass circle that the 

 latter comes to rest in an initial position, and anj^ disturbance from this 

 position is shown by the motion of an index. The lower half of the cir- 

 cular glass tube is filled with mercury, and the differential expansion of 

 the air in the two bulbs, by altering the position of this mercurial col- 

 umn, causes the entire apparatus to rotate around the axis of the circle. 

 {Z. O. G. M., Vol. XV, 1880, p. 493.) 



Pictet and Cel6rier have constructed a form of thermo dynamometer 

 which can be used as a very sensitive self-recording thermometer. The 

 thermometric substance adopted in this instrument consists of a saturated 

 vapor of some volatile liquid, which substance varies according to the 

 temperature that is to be measured. Thus, for a range of — 40 C. to 

 +25 C, pure anhydrous-sulphuric acid ; for the range +25 C. to +90 C, 

 sulphuric ether; for the range +90 C.to +200 C, distilled water. These 

 fluids are introduced into an inclosed space, L; the vapors press upon 

 the mercury in the manometer at M; and the tension of the vapor is 

 shown by the height of the mercurial column MM. The tension of the 

 vapor depends upon the temperature of the mixture of fluid and vapor, 

 as shown by the following equation : 



loo- ,,nt f"P'\ ^'+{o-^ ) (t'-t) 431x1.293 0-27 4 jt'-t) 

 *• "'^^-^p J- 10333 (274+ «') (274+*) 



In this equation P and P' are the vapor tensions corresponding to the 

 two temperatures t and t', of which t is the temperature to be measured 

 and t' an arbitrary constant temperature; c is the specific heat of the 

 fluid, and 1c the specific heat of its vapor; 8 is the variable density of 

 the vapor; gfr is the coefficient of the expansion of gases; 10333 is the 

 pressure on a square meter of a column of mercury 760 millimeters high ; 

 431 is the most probable value of the mechanical equivalent of feet as 

 deduced by Pictet from Eegnault's data. 



In the apparatus constructed by these authors for the observatory of 

 the city of Geueva, the ordinary range of temjjerature, — 20 0. to +40 

 C, is represented by a motion of the mercurial column of more than 4 

 meters, which is represented upon the graphic paper record by the mo- 

 tion of about one-half a meter. {Z. 0. G. M., XIV, p. 248.) 



The hygrometer designed by Edelmanu, and constructed at his phys- 

 ical mechanical institute at Munich, depends upon the principle that 

 when a given space is filled with moist air, and the vapor is absorbed 

 therefrom without altering the volume, then the pressure diminishes by 

 quantity equal to the tension of the vapor contained in the air. The 

 principal portion of Edelmann's ai)paratus consists of a horizontal cylin- 

 der, closed at both ends with corks, through which pass tubes for the 

 entrance of the air outside, and which also connect with the manometer. 

 The method of making a measurement is as follows: 



First, make the entire apparatus clean and dry; second, j^lace the 



