Intelligence and Miscellaneous Articles. 233 



A2J AIR-THERMOMETER WHOSE INDICATIONS ARE INDEPENDENT 



OF THE BAROMETRIC PRESSURE. BY ALBERT A. MICHELSON. 



The appearance of an abstract of a paper by Pettersson* on a 

 new air-thermometer has led me to publish, sooner than I had 

 contemplated, a notice of an instrument far simpler and more 

 manageable than that which is there described, and which likewise 

 retains the important advantage of giving indications which are 

 independent of the external pressure. 



The instrument consists of a glass bulb and stem, the former 

 about 40 millim. and the latter about 2 millim. in interior diameter. 

 The bulb contains dry air at a pressure of about 100 millim. of mer- 

 cury : and this air is separated from the upper portion of the tube 

 by a column of mercury about 100 millim. in length. The mercury 

 remains above the air, notwithstanding the large diameter of the 

 bore, owing to the resistance to deformation of the meniscus. The 

 space above the mercury is a vacuum. 



Thus the pressure of the air in the bulb is constant, and is equal 

 to that of the column of mercury above it. If the bore of the stem 

 is not of uniform section, the length of the column will change ; but 

 this length is easdy read off, and gives at once the true pressure. 



The pressure need not be limited to 100 millim.; but if it be much 

 greater the instrument becomes inconveniently long. 



The only precaution to be observed, beyond what is used in an 

 ordinary mercurial thermometer, is that the stem must be kept ver- 

 tical. — Silliman's American Journal, August 1882. 



Case School of Applied Science, 

 Cleveland, O., July 5, 1882. 



ON A PROPERTY OF THE ISENTROPIC CURVE FOR A PERFECT GAS 

 AS DRAWN UPON THE THERMODYNAMIC SURFACE OF PRESSURE, 

 VOLUME, AND TEMPERATURE. BY FRANCIS E. NIPHERf- 



The equation of this thermodynamic surface is 



_pv = KT; (1) 



where p, v, T represent the pressure, volume, and absolute tempe- 

 rature, and where E is directly proportional to the volume of a 

 unit mass (or inversely proportional to the density) of the gas at a 

 standard temperature and pressure. 

 By differentiation, (1) becomes 



dp=^dT- ^dv (2) 



Tor convenience, putting 



^=A. ?I=B. 



v ir 



(2) becomes 



dp=AJI-Bdv (3) 



1. To find the direction of maximum slope with respect to the 

 v, T plane at any point on the surface. For this purpose pass a 

 * Annalen der Physik v.nd Chemie (Beiblatter), Xo. 5, 1882. 

 I From Trans, of St. Louis Academy of Sciences, read April 3, 1882. 



