208 On the amount oj Rain -fall at Calcutta. [April, 



It will be observed that the numbers in the General Mean (the last 

 column) are somewhat irregular, which I apprehend is owing to the 

 series of years being too short for the subdivision I have adopted, viz. 

 2°. 30', if instead of that we take 5° as the subdivision, the numbers 

 come out regularly, as follows : 



Moon's declination. General Average of Rain-fall. 

 5 degrees. *321 inch. 



10 do. -271 



15 do. '256 



20 do. -259 



25 do. .347 



The results are somewhat different from what I expected, for they 

 shew an increase of rain, not only towards the maximum, but towards 

 the minimum declination of the moon. Had it been towards the 

 maximum only, we might have accounted for it by supposing the rain 

 to vary with the principal tide, either superior, or inferior ; and had it 

 been towards the minimum only, we might have supposed that the 

 rain was the effect of the mean tide, as in all latitudes, less than 45°, 

 the mean tide increases as the declination of the moon diminishes. 

 However, when our data are more perfect, we may be able to get an 

 explanation of the phenomena. In the meanwhile, lest any one 

 should object that the series of years for which the average has been 

 taken, is too short to establish the fact of an increase towards the 

 maximum declination, I beg now to offer some other reasons which 

 led me to the conclusion before I obtained a sight of the Almanacks. 

 I must first remind you that, owing to the revolution of the nodes 

 of the moon, her maximum monthly declination decreases for a series 

 of years, and then increases. Thus if we turn to the Table, we find 

 that in the year 1829, and for two years both before and after it, the 

 maximum declination was always less than 20°. This revolution of the 

 nodes is completed in a period of about 18f years, or more correctly, 

 6803 days, 2 hours, 55 minutes. Now then, supposing it to be true that 

 the rain-falls vary with the declination of the moon ; in those years in 

 which the declination is small the rains ought to be scanty, and vice 

 versd to increase as the former increases. We have no register of 

 rain for a long series of years, but we have a valuable record left us 

 for the illustration of this part of our subject, similar to that register 

 of the height of the annual inundations of the Nile, which the ancient 

 Egyptians measured by means of a NeiKoo-noireiov, or Nilometer, placed 

 on the bank of the river ; I allude of course to Mr. Kyd's Register of 

 the height of the Hooghly in different years*. In the map No. 4, 



* See his paper on the subject, (Part 1. Trans. Phys. Class, As. Soc.) and 

 the map which accompanies it. 



