Intelligence and Miscellaneous Articles. 159 



meter is identical with that which would be exerted by a disk of 

 surface w placed at the aperture of our sphere, this disk having the 

 same temperature and emissive power as the sun. We can there- 

 fore define the temperature of the sun by that which would have to 

 be attributed to this imaginary disk, possessing the emissive power 

 of lampblack, to produce upon the thermometer the same effect 

 which is actually produced by the sun. Let w be the temperature, 

 thus denned, of the sun, the stationary temperature of the ther- 

 mometer receiving the solar radiation through the aperture w ; the 

 quantity of heat emitted by the thermometer (which was S«* at the 

 temperature t) has become $a 9 ; and putting that quantity of heat 

 equal to the sum of the quantities emitted by enclosure and by the 

 sun, we have at once 



This is precisely the equation as written by M. Vicaire ; but it was 

 established under reserves from which we must now free ourselves. 

 The dimensions of the thermometer are necessarily finite ; and con- 

 sequently the aperture through which the solar rays penetrate must 

 be widened to permit them to reach the whole of the bulb : hence 

 comes a double complication. 



Let us now consider an admission-aperture £2 large enough for 

 an entire hemisphere of the bulb to receive the rays of the sun. If 

 the diameter of the bulb is sufficiently small in proportion to that of 

 the enclosure, every point of it will be sensibly in the same condi- 

 tions ; so that in order to account for the actual state of the appa- 

 ratus, it is sufficient to consider any one point whatever of the 

 bulb. This point is submitted : — (1) to the radiation of all the 

 preserved portion of the enclosure ; (2) to the radiation of the sun, 

 which is equivalent to that of a surface w placed at a distance equal 

 to the radius of the enclosure and kept at the temperature of the 

 sun ; (3) to the radiation of the whole of a portion of the sky bor- 

 dering the sun, which acts as a surface O — io at an unknown tem- 

 perature y. The precise equation is, therefore, 



Sa* = Ha f + ua* + QaV. 



I will indicate in a forthcoming note how, making O to vary by 

 means of diaphragms pierced with apertures of known dimensions, 

 the correction-term Qrt# can be determined with sufficient exact- 

 ness. An idea of its quantity will be given by the following result, 

 the only one I shall cite at present : — 



On March 14, 1874, the sky being very clear, although the ground 

 was covered with snow, at 1 p.m. the quantity of heat arriving from 

 the sun at the surface of the ground was the same as that which 

 would have been given by a disk of the same apparent diameter as 

 the sun, of maximum emissive power, and at the temperature of 

 1238° C. The temperature of the air was +1°, and the barometric 

 pressure 758 millims. In these conditions, the diameter of the 

 admission-aperture being about 25 times the sun's apparent dia- 

 meter, the portion of the sky bordering the sun, and seen from the 



