1834.] Atmospherical Phenomena. 347 



mum in the four periods. In the early part of the year, the days imme- 

 diately succeeding it are maxima, and in the latter part of the year the 

 days immediately preceding it. If therefore any cause can be assigned 

 why the rain at one time should be a little retarded, and at another a 

 little accelerated, there would be no error in supposing the maximum 

 tendency to rain to occur on the 9th day. Two other days of maxima 

 are the 3rd day before the new moon, and the 12th day after. 



With a view of ascertaining whether the Barometer was similarly 

 affected, I next put the heights of it at sunrise for five years into a simi- 

 lar table, and on taking the mean of the whole year, found that the fifth 

 day after the new moon was the minimum. (See Table No. 3). My 

 next object was to find whether the dew points varied in a similar man- 

 ner, and I therefore reduced them from the wet-bulb indications in the 

 registers, and those of Leslie's hygrometer preceding them. The me- 

 thod recommended in the 1st volume of Gleanings being too laborious 

 to adopt, and Major Oliver's tables in the Gleanings not having been 

 published, I took the very simple method of multiplying the wet-bulb 

 depression by 1*6, and subtracting the product from the temperature*. 

 I do not mean by saying so to recommend the operation as a general 

 rule, but only that, where the whole difference between the temperature 

 and dew-point does not amount to more than two or three degrees (as is 

 usually the case at sunrise at Calcutta), and the temperatures are between 

 80 and 50, the errors will not be important. But another and greater 

 difficulty still occurred to me, which was this. By a copious fall of rain 

 the dew point is immediately lowered, so that on looking over the list of 

 dew points on different days, the day of most moisture will appear to be 

 the driest by its having the lowest dew-point. Thus, for instance, in 

 May, 1830, they stood as follows : 



Days of Month. 22nd, 23rd, 24th, 25th, 26th, 27th, 28th. 



Dew-points at") 72'9 78-2 74-9 75-1 75-6 74.9 77.9 



sunrise, / 



Rain-fall, 0-10 3.00 4-22 



Here the 25th and 26th were the days of the great storm, but looking 

 atthe dew-points alone, no indication is afforded of the quantity of mois- 

 turef. Unless, therefore, we could make due allowance for the rain- 

 fall, the dew-points alone would be a very imperfect mode of judging 



* This rule would answer for an aqueous tension of 0.75 at the temperature of 

 90°. We think it would have been better to have used the aqueous tensions them- 

 selves, for which a table is given in the Gleanings, I. p. 81 and 340. — Ed. 



t When rain is accompanied with a strong wind, and that from the north, 

 the air is seldom saturated with moisture ; the chief cause however for the fall of 

 the dew-point is, the reduced temperature of the air during storms. — Ed. 



Y Y 2 



