234 



Intelligence and Miscellaneous Articles, 



The following is the course of an experiment : — All the tubes 

 being carefully closed, and the thermometer in place, the tempera- 

 ture (which is stationary if all has been well regulated for a suffi- 

 cient time) is read ; then the admission-tube is opened after bring- 

 ing opposite to it such aperture of the diaphragm as is judged 

 suitable. Now, the apparatus being kept in accurate orientation, 

 we wait until the temperature again becomes stationary, and then 

 note the excess shown by the thermometer. 



Experiment shows that this excess depends both on the thermo- 

 meter employed and on the diameter of the aperture of admission. 

 No precise conclusion, therefore, can be drawn from experiments in 

 which we have not preoccupied ourselves with the dimensions of 

 the thermometer, and with the magnitude of the admission-aperture 

 pierced in the enceinte, with the temperature constant. On the 

 contrary, by employing in succession different thermometers and 

 different apertures of the diaphragm, we can evaluate very accu- 

 rately : — (1) the cooling due to the contact of the air ; (2) the heat- 

 ing which proceeds from the radiation of the portion of the sky 

 bordering the sun and seen at the same time from the bulb of the 

 thermometer. I shall show this by an example, the data of which 

 I take from one of my last series of observations. 



On the 20th of June last, operating successively with two ther- 

 mometers, the spherical reservoirs of which had the diameters 12 

 millims. and 7 millims. respectively, and with three different aper- 

 tures a, b, c of the diaphragm, the respective diameters of which 

 were 17'5, 14'5, and 12 miUims., I obtained the following results : — 



Time. 



Temperature of 

 the enceinte. 



Temperature of the 

 large thermometer. 



Temperature of the 

 small thermometer. 



h m 

 2 40 



2 55 



3 10 

 3 30 



3 45 



4 10 

 4 20 

 4 35 



1410 

 1405 

 1405 

 14 00 

 13-95 

 13-90 

 13-85 

 13-80 



27-03 (diaphragm a) 

 2656 (diaphragm b) 



24-05 (diaphragm b) 

 23-63 (diaphragm c) 

 23-85 (diaphragm a) 



23-43 (diaphragm a) 

 23-30 (diaphragm a) 







2605 (diaphragm a) 



Let us take first the observations of 2 h 55 m and S h 10 m ; these 

 two, made nearly at the same time, should lead to sensibly equal 

 excesses of temperature. The considerable difference between the 

 two numbers observed arises from the complication introduced into 

 the experiment by the presence of air : to the radiation from the 

 bulb of the thermometer is added the cooling produced by the air ; 

 and in these two ways the bulb loses a quantity of heat equal to 

 that which it receives from the sun. The loss of heat in vacuo, 

 making equilibrium with the same quantity of heat received from 

 the sun, would therefore be equal to the loss observed plus the loss 

 due to the air. But, according to Dulong and Petit, the lowering 



