48 Meteorological Ohservations. [Jan. 



pression caused by radiation during the niglit. I need 

 scarcely remark that this will not give such exact results aS 

 the more perfect instrument described above. 



Photometer. — Of the various contrivances, for mea* 

 suring the intensity of the solar rays, none are equal, in 

 accuracy to the photometer of Leslie. — This is a modification 

 of his differential thermometer, one of the balls of which is 

 blown of black glass, while the other is formed quite pellu- 

 cid. The rays of light, which fall upon the clear ball, 

 therefore, pass through it, without suffering obstruction ; but 

 those which strike the dark ball are absorbed at its surface, 

 and excite or communicate heat. This heat must hence 

 accumulate, till its subsequent dispersion through the air 

 comes to balance exactly its constant accession. The 

 space through which the coloured liquid sinks in the stem will 

 thus measure the momentary impressions of light, or its ac- 

 tual intensity. — To prevent any extraneous agitation of the 

 air from accelerating the discharge of heat at the surface of 

 the black ball, and thereby diminishing the full effect, the 

 instrument is always sheltered, more especially out of doors^ 

 by a thin glass case." 



The same objection being applicable to this elegant in- 

 strument as the CEthrioscope of Leslie, I have been induced 

 to substitute for it the following contrivance, which, al- 

 though not so sensible to small impressions of light will be 

 found to be fully as accurate. A thermometer, (Fig. 5.) with 

 a millesimal scale, has a black opaque ball, over which is 

 placed a glass cover fixed on by means of a screw. The 

 difference between this instrument, when exposed to the 

 sun, and one placed in the shade, will be the measure of the 

 heat due to the rays of light. 



Few accurate observations have hitherto been made, to 

 ascertain the intensity of the solar rays, in different seasons 

 of the year, and in different climates.* — This is a branch 

 ©four subject, which presents a wide field for interesting 

 research; and much curious information respecting the 

 difference in the intensity of the solar rays, in the plains, 

 and on the mountains of India, will probably be obtained by 

 the employment of the instrument I have just described. 



* Consult Daniel's Meteorological Essays. 



