Observations on some Experiments in Electricity. 61 
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 
302°, while the mean dew-point, for the same months is 233°, 
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 epetine neg see at 
each season. Now, a cubic inch of air at 232° conta of 
a grain of moisture, and the same bulk at 61.10° contains are rnees 
gr. or the latter hasnearly three and a half tumes as muchas 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 spring of the year while both the temperature and the dew- 
point ar dually rising,—but the former, of course, rising most rapid- 
acti distance of the two is 124°; and inthe three autumnal months, 
when the temperature is descending, and the dew-point consequently 
falling, but with less rapidity, the distance between them is only 8.549. 
The mean temperature in the months of March, April, and May is 
57.66°, and in September, Ocfober, and November, it is 55°. So that 
* See Journal of the Franklin Institute, Vols. vii, viii, ix, and x- 
