1814.] Scientific Intelligence. 313 



happened during a heavy shower of rain on the 13th of May, 1813. 

 I do not pretend to be able to solve this problem scientifically; but, 

 as from the manner of stating it you seem to have left it open to 

 observation, I beg leave to hazard a few conjectures by way of ex- 

 planation. 



The thermometer, it appears, stood at 60° about nine o'clock 

 in the morning immediately before the shower, arid while the rain 

 was falling it sunk rapidly to 50°, and never, during the remainder 

 of that day, regained its former elevation. 



It may, I think, be remarked, that the temperature of 60° was a 

 degree of warmth by no means inconsiderable for that period of the 

 year, and at that hour of the morning. The rays of the sun must 

 have exerted a powerful influence, and the surface of the earth must 

 have been a good deal heated. That being the case, the clouds, or 

 that portion of vapour, the subsequent condensation of which occa- 

 sioned the shower, must have been suspended at too great a height 

 in the atmosphere to produce, by the caloric given out, any very 

 sensible change in the temperature near the surface. The imme- 

 diate consequence of the shower falling is the commencement of 

 the process of evaporation, which continues during and after the 

 shower, till the whole, or nearly the whole, of the moisture is dissi- 

 pated, and then the thermometer resumes nearly its former alti- 

 tude. The effect of evaporation is the absorption of caloric and 

 the generation of a great degree of cold, which would, under those 

 circumstances, be rendered sensible by the thermometer. Although 

 the power of the solar rays might be great as soon as the rain had 

 ceased, and though the heat extricated in the upper regions of the 

 atmosphere might extend to the surface of the earth ; yet its in- 

 fluence is comparatively so limited, and the evaporation so consider- 

 able, that it would not be felt. It is, I believe, universally ob- 

 served, that in summer, when the heat is great, a heavy shower of 

 rain < rauses a reduction of temperature, or a coolness of the air; and 

 1 own 1 can at present imagine no way in which it can do this, ex- 

 cepting the one I have pointed out. At all events, the surface of the 

 earth being heated, and the temperature of the water contained in 

 the shown being necessarily lower than that of the earth, it follows 

 of course that, whether there be an immediate evaporation or 

 not, there must a prut of the caloric at the surface pass into the 

 water, and become united with it. This of itself will occasion a 

 reduction of the sensible heat. 



Upon principles nearly the reverse of the former is probably to 

 be explained the increase of the temperature which generally fol- 

 low- a vhowei of rain in winter, or during a long tract of very cold 

 it her. Then the clouds, whose condensation is to form the rain, 

 maintain, by reason of the diminished temperature at the surface, a 

 much leas elevated position in the atmosphere. As soon as the 

 condensation takes place, caloric is disengaged, and, from the low 

 stat( of the temperature near the surface, the influence of this ca- 

 • is immediately felt, and gives rise to that augmentation of the 



