Dr. Buriicy on the Strength of the Suiih Rays. 5 



any cover. On examining the glass tube, it was ascertained 

 that the air within it had acquired a suflicient temperature be- 

 yond the free surrounding air to condense, wliereby fine dew 

 or vapour steam was deposited all over the inner surface of 

 the tnbe, so that the thermometer was obliged to be taken 

 out, in order to see its height ; when it was found to coincide 

 with the temperature of spring water, as taken at eight o'clock 

 A.JNi., viz. 49°. The dew or vapour steam was solidly frozen in 

 the glass tube in three minutes after it was placed in the shade. 



This experiment points out the great necessity of keeping 

 the bulb of the thermometer uncovered in ascertaining the 

 strength of the solar rays, which it is presumed has not been 

 nicely attended to in similar experiments recently published. 

 It also in some measure shows the manner in which dew is 

 formed on the inside of glass windows, and transformed into 

 icy efflorescences, by the union of the exterior air with that 

 in the tube, or by the inosculation of the two airs of unequal 

 temperatures. 



The second experiment, after the glass tube was dried again, 

 fell 5° short of the first ; but the sun having passed the meri- 

 dian upwards of an houi', the strength of his rays, or the tem- 

 perature of the air in the tube, was consequently so much di- 

 minished. 



It should also be observed, that in the first experiment the 

 thermometers in the sun's rays were suspended by a string, so 

 that no radiations from near objects, nor any obstructicn of 

 the free air, should have the least tendency to raise them 

 higher : but without this necessaiy pi'ecaution they were 3" or 

 -4° higher. 



By similar experiments made with the same thermometers 

 suspended in a meridian sunshine at the beginning of last July, 

 it was ascertained that the power of the solar rays was from 

 25° to 30° greater than the temperature of the contiguous 

 shaded air: and since then it has been found by interven- 

 ing experiments, that this power diminishes pretty uniformly 

 (having some regard to the position of the wind at the time) 

 with the increase of the sun's south declination. That so 

 great a diminution, we will say within the mark, as ^ths of 

 solar influence should be lost to us in this latitude at this sea- 

 son, compared with the opposite one, is natural, when we re- 

 flect for a moment upon the effects of the solar rays through- 

 out the seasons of the year, and their unparalleled benefit to 

 the inhabitants of the various parts of the earth, according to 

 its progressive annual motion round the centre of our system, 

 the sun. I am, yours, &c. W. Buuney. 



III. On 



