488 Queries and Answers. 
phenomena, and variable according to natural circumstances, or is it only an 
indication arising from the mechanism of the instrument ? We know that 
any solid body, presented to warmer air, begins and continues to be moist- 
ened till its temperature is equal to that of the air. Now, which degree of 
this increasing moisture on a body cooled down by artificial evaporation can 
be properly called “ the dew-point ?” It begins (whether visible or not) at 
the second degree below the temperature of the air, and continues as long 
as a lower degree of heat can be obtained by evaporation, how then can any 
degree of this process be fixed on, to be called the dew-point ? Again, what 
is inferred from the appearance of moisture on the cold body : does it show 
the quantity of water then in combination with the air, i. e. the degree of 
saturation ; or does it indicate the solvent power of it ? conceive there are 
three very distinct states of the air; it is either taking up, is saturated, or 
letting go the water with which it enters into combination. The indications 
of instruments invented to mark these different states of the atmosphere 
should therefore be described in proper terms; a lower temperature pro- 
duced by evaporation should not be attributed to radiation, nor should any 
degree of the solutive power of the air be called a degree of saturation. 
The “ Remarks on the Weather” in the Companion to the Almanac, pub- 
lished by the Society for the Diffusion of Useful Knowledge, are obscurely 
written. In one place (p. 17.) it is stated that, by some philosophers, the 
increase of the weight of the air has been supposed to proceed from the 
quantity of water dissolved in it; “ but this is refuted by the simple fact, 
that when the barometer stands highest, the air is most dry.” Now, had 
this sentence been written, when the barometer stands highest, the air is most dry- 
ing, it would have been equally intelligible, and, I will venture to say, more 
consistent with fact. In p. 19., in describing the indications of the hygro- 
meter, it is said, that “a rise in the dew-point, accompanied by a fall of the 
barometer, is an infallible indication that the whole mass of the air is 
becoming imbued with moisture, and copious precipitations may be looked 
for.’ Unluckily for the novice in such matters, it is not added whence 
this copious precipitation is supplied; and if such indications follow a 
period of fine weather, as they necessarily do, the puzzle is still greater. 
I regret to observe, too, in a paper “ on the atmosphere,” in a late number 
of a celebrated agricultural work, that effects are attributed to assumed 
causes, not at all creditable to the writer as a meteorologist. 
Your correspondent, I hope, will excuse my proposing this subject to his 
attention : he appears to think for himself, and I trust he will oblige your 
readers with his ideas on it. — J. Main. Chelsea, May 7. 1829. 
Why Silks and Flannels are apt to emit Sparks in Frosty Weather, §c. 
(p. 200.) — I am not aware that si// is more easily excited, so as to pro- 
duce electrical phenomena, in frosty weather, than any other electric. It is 
well known that air is a very imperfect conductor when dry; but the elec- 
tric fluid is readily transmitted through moist air. In frosty weather, when 
the air is dry, it becomes, as it were, an Insulator round the silk or other 
electric. Hence the electric fluid, being confined to and round the surface 
of the excited body, exhibits the phenomena of light, attraction, and that 
phenomenon which has generally been termed repulsion. M. expresses a 
wish that some of your correspondents would send a communication, for 
insertion in your Magazine, upon the subject of electricity. If consistent 
with your plan, I shall be happy, as far as I am able, to comply with his 
request. — A. L.A. Alnwick, April7. 1830. 
The subject of electricity we would rather decline, as it belongs more to 
natural philosophy than to natural history. — Cond. 
