METEOROLOGY. 439 



dne to thin successive layers of varying density at different heights. 

 {Nature, xxvi, 1C7.) 



In a third communication on mirage, Professor Tait called attention to 

 an elaborate memoir by Biot, who had anticipated him in the theory of 

 the curve of vertices, but had not made any further use of it. Other 

 points at issue between Biot and Tait can perhaps be settled by care- 

 ful measurements of the dip of the horizon taken at different heights 

 above sea-level. {Nature, xxvi, 264.) 



Fonvielle states that during the month of January, 1882, a large tract 

 of country, including Paris, was persistently covered by an obscure 

 cloud, or nebulosity, such that neither sun, moon, nor stars were visi- 

 ble from the 4th to the 26th. French meteorologists were of the opin- 

 ion that this was due to a mass of snow suspended in the atmosphere, 

 but as he entertained a different opinion he determined to test the ques- 

 tion by ascending in a balloon, which he did on January 25, when he 

 found that not a single flake of snow was present, and that moreover 

 the thickness of the cloud did not exceed 1,000 feet, although it ren- 

 dered the sun perfectly invisible from the earth. When at a height of 

 900 feet Fonvielle found the earth in its turn invisible, and at the height 

 of 2,000 feet, the cloud having been passed, and the sun shining in a clear 

 blue sky, this nebulous matter appeared to him perfectly homogene- 

 ous and without traces of crystals of snow. The temperature within the 

 cloud was about 41° F., but above the cloud, about 28° F. Hoar-frost 

 formed on the balloon when above the cloud, but not when within it, and 

 he ascertained that the cloud was really formed of microscopic atoms of 

 water in a quiescent state, and he thinks that when such atoms are set 

 in motion they crystallize into minute spiculse or hairs, and that these 

 observations go far to explain the formation of "cirrus" clouds. {Nature, 

 XXV, p. 338.) 



Kopp, in reference to Fonvielle's observations of the nature of the 

 nebulous matter floating over Paris, states that he thinks it more prob- 

 able that the cloud was formed by small drops of liquid water cooled 

 below the freezing point, but not crytallized or even solidified until they 

 come in contact with a solid body, when the surface tension of the drops 

 is immediately diminished, and crystallization ensues. " We know," he 

 says, "from Dufour's observations, that water-drops, if they are not in 

 contact with solid matter and are floating in a mixture of oil and chlo- 

 roform, and having the same density, may be cooled down to — 10 C, 

 and if they are small enough, even to — 20 C. Hoar-frost formed dur- 

 ing hazy weather and with a temjicrature below freezing may be due to 

 the solidification of such drops of mist. {Nature, XXV, p. 385.) 



In reply to the above, Fonvielle states that as he. saw no signs of a 

 rainbow he cannot admit that there were any liquid water-drops in the 

 cloud. {Nature, xxv, p. 430.) 



Kopp replies to the preceding, that no rainbow should have been 

 visible, for the minute particles were not rain-drops, but the minutest, 



