488 Queried and Ans'uoers. 



phenomena, and variable according to natural circumstances, or Is it only an 

 mdication 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 ? I conceive there are 

 three very distinct states of the air j 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 j 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- 

 ingy it would have been equally intelligible, and, I wUl 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. ChelseOy May 7. 1829. 



Why Silks and Flannels are apt to emit Sparks in Frosty Weather^ S^c. 

 (p. 200.) — I am not aware that silk 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 readUy 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 , April!. 1830. 



The subject of electricity we would rather decline, as it belongs more to 

 natural philosophy than to natural history. — Cond. 



