NATURAL PHILOSOPHY. 151 



of water in the air which would saturate it as divided into 100 parts, 

 and then ascertaining how many of these parts are present. From all 

 the experiments treated in this way, the laws of moisture thus expressed 

 are, with an overcast sky, almost uniform degree of humidity to the 

 height of 3,000 feet or 77 out of the 100 parts then a rather sud- 

 deif decrease to 80, and to 83 at 5,000 feet. With a partially clear 

 sky, the laws of moisture show a humidity on the ground, with 15 out 

 of 100 parts more than in partially clear skies, and of 14 at 5,000 feet. 

 Above 5,000 feet, the humidity decreases to 10 at 25,000 ; that is, low- 

 as the temperature there is, 10, out of the 100 possible to be present, at 

 the temperature, is all that is present. Higher than this, there would 

 seem to be an almost entire absence of aqueous vapor. These seem 

 to be the general laws ; but, as I have before remarked, the regular 

 dimensions are frequently interrupted, and strata of moist air may ex- 

 ist at great elevations. 



As regards the blue color of the sk?j, Mr. Glaisher was inclined to at- 

 tribute the phenomena to reflection of light from the molecules of air, 

 and not, as some have supposed, to reflection from the thin pellicle of 

 water forming the vesicles of vapor floating in the air ; inasmuch as 

 the blue was found to be brightest at the greatest heights (six and 

 seven miles) attained to by the aeronaut, where the air is almost de- 

 prived of moisture. 



In an ascent made in a rain-storm on the 2 7th of July, Mr. Glaisher 

 directed his attention particularly to observe whether there was a 

 stratum of cloud at a certain elevation above that from which the rain- 

 drops fell ; and also as regards the size of the drops at different eleva- 

 tions. The conclusions arrived at were, that whenever rain is falling 

 from an overcast sky, there is a second stratum above ; but with an 

 overcast sky and no rain, then the sun is shining on the upper surface 

 of the clouds. In regard to the second point, he says, " The size of 

 the rain-drops, as they fell on my note-book before starting, was fully 

 as large as a four-penny piece ; they decreased in size on ascending ; 

 but our upward movement was too quick, and we soon passed out of 

 rain. On descending from above the clouds, we first encountered a 

 dry and then a wet fog ; passed into that which may be described as 

 damp air or exceedingly fine rain ; then experienced very fine but de- 

 cided drops of rain, like pins' points, covering the note-book ; these in- 

 creased in size on approaching the earth, but more rapidly when very 

 near the earth. The drops of rain, on returning to the earth, were as 

 large as those noted on leaving ; and rain had been falling heavily all 

 the time we were in the balloon." Another curious fact elicited in 

 these ascensions is that the action of the sun's rays upon " sensitized" 

 photographic paper is much less at great altitudes than near the earth's 

 surface. In an ascension made April 21st, 1863, Mr. Glaisher took 

 with him slips of such paper, and arranged that similar slips should be 

 exposed at Greenwich Observatory, and the amount of coloration not- 

 ed simultaneously every five minutes. In his report, he states that the 

 paper in the balloon was exposed to the full rays of the sun, with this 

 extraordinary result, that at three miles high the paper did not color 

 so much in half an hour as in the grounds of the Royal Observatory in 

 one minute ; a fact which would seem to indicate that the chemical 

 eifects of li^ht are largely due to its passage through the atmosphere, 



