1834.] on Atmospherical Phenomena. 349 



the moon in such a position as to act in conjunction with it, then the 

 two forces would produce a great conjunction tide in the air. As the 

 moon recedes eastward from the sun, it comes upon the meridian about 

 48| minutes later every day ; so if the change happened at noon ex- 

 actly, three days after it would be upon the meridian at 2h. 25m. p. m. 

 As the time of maximum heat of the day is by Dr. Brewster 2h. 40m, 

 p. M.we ought on this day to have the great conjunction tide, according 

 to theory. But in comparing the actual tides of the ocean with the de- 

 ductions from theory, we find that the phenomena occur one day and a 

 half later than they ought to do ; thus the greatest spring tide does not 

 happen exactly at the conjunction of the sun and moon, but a day and 

 a half later. Let us make a similar allowance in the case we are con- 

 sidering ; then as the moon must be somewhat more than three days 

 old when it is upon the meridian at 2h. 40m. p. m. add one day and a half 

 to its age, and the greatest tide will be produced when it is nearly five 

 days old. I venture to suggest this as the cause of the maximum fall of 

 rain on the fifth day after the new moon, and the minimum of the 

 barometer on that day. Of course, as air is distended and rendered 

 lighter by being mixed with aqueous vapour, the presence of a great 

 quantity of moisture (which would be the case in a current setting in 

 from the ocean) is sufficient to account for the diminution of pressure. 



Cor. 1st. This supposition may account for our spring showers 

 happening as they usually do between 2 and 6 p.m. and probably at 

 the time when the superior or inferior tides of the moon are near the 

 meridian. 



Cor. 2nd. If the supposition be true, then the excess of rain about 

 the fifth day after the new moon will be greatest, when the heat is a 

 maximum, when the sun is nearest the zenith, and when the moon is 

 nearest the zenith. This would happen at Calcutta in the months of 

 May and June. In the first four months the heating power is great, 

 but the sun has south declination for most part of the time, and the 

 moon too. In July the sun is near the zenith, and so is the moon, but 

 the heating power is counteracted by constant evaporation. In the 

 last three months of the rains the sun and moon are further from the 

 zenith, and the heating power somewhat less than in July. I took 

 therefore the sum of the rain that fell on the 3rd, 4th, 5th, 6th, and 7th 

 days after the new moon, in each of the four periods, and compared each 

 with the quantity that fell during the whole lunar period. Premising 

 then that five days are to the whole lunar period as 10 : 59 



I found the sum of rain that had fallen in the 5 days above men- 

 tioned, to be to the sum of the whole lunar period, — 



