Observations on some Experiments in Electricity, 01 



absence of means to raise the dew-point of the heated apartment ; 

 otherwise, the warmer air will not long avail towards insulating the 

 charges. I cannot at present state how near the two points above 

 mentioned may approximate without entirely destroying the power 

 of the machine to retain the electricity which it develops ; but when 

 they are only 6° or 7° Fahr. from each other, its action will be com- 

 paratively feeble. The effect of their approaching within that num- 

 ber of degrees, will, however, be found more injurious in summer 

 than in winter, because the absolute quantity of water in a given bulk 

 of air, at the dew-point of the former season, is much greater than 

 is found at that of the latter. 



From observations* made at Philadelphia during two years prece- 

 ding the month of May, 1833, it appears that the mean temperature 

 of the months of December, January, and February, is at this place 

 30|°, while the mean dew-point, for the same months is 23f °, 

 or seven degrees below. The mean temperature in June, July, and 

 August, is 72.17°, and the mean dew-point, 61.10°, or 11.7° 

 below ; so that there is a difference of more than 5° in the dis- 

 tance of these points from each other, in summer and in winter. But 

 in order to know the degree of deterioration to which the action of a 

 machine will be liable from the hygrometric state of the air, we ought 

 perhaps to consider also the absolute quantity of moisture present at 

 each season. Now, a cubic inch of air at 23f° contains .00103359 of 

 a grain of moisture, and the same bulk at 61.10° contains .00350707, 

 gr. or the latter has nearly three and a half times as much as the former. 

 It is probable, then, that if we were to make experiments in the open 

 air, both in summer and in winter, the greater excess of temperature 

 in the one, would about counterbalance the less moisture in the other; 

 but, as we can generally employ an artificially heated apartment in 

 winter, which personal comfort would preclude in summer, we more 

 frequently operate at that season under favorable circumstances than 

 in the warm portion of the year. 



In the spring of the year while both the temperature and the dew- 

 point are gradually rising, — but the former, of course, rising most rapid- 

 ly, — the distance of the two is 12^°; and in the three autumnal months, 

 when the temperature is descending, and the dew-point consequently 

 falling, but with less rapidity, the distance between them is only 8.54°. 

 The mean temperature in the months of March, April, and May is 

 57.66°, and in September, October, and November, it is 55°. So that 



" See Journal of the Franklin Institute, Vols, vii, viii, is, and x. 



