METEOROLOGY. 310 



of festooned or poclcy clouds, such as Poey proposes to call gloho-cunm- 

 lus. (Nature, xxviii, [). 79.) 



208. Mr. John Aitkeu and Dr. O. L. Lodge have made inimerous ex- 

 periments to elucidate Ibe reasons for the so called clear spaces which 

 are absolutely free from dust in the neighborhood of solid bodies (the so- 

 called dark plane of E*rofessor Ty udall and Lord Kaleigh). They conclude 

 that the dust is repelled from hot surfaces in the same way as the vane 

 of a Crookes radiometer is repelled when placed in front of a hot sur- 

 face, namely, one hotter than the intervening gas, the principal part of 

 the energy of the motion being given to the rej^elled surface or dust 

 particle by the kinetic energy of the impinging molecules; the attrac- 

 tion of a cold surface is explained by the less kinetic energy of the out- 

 ward-going molecules of the hotter gas. A wet and hot surface repelled 

 dust twice as strong as a hot, dry surface. Mr. Aitken, led by these 

 views, constructed a dust tra]) by means of a tall metal tube surrounded 

 l)y another larger. The smoke goes up the center tube and down the 

 intervening space; the soot is deposited on the cool inside of the outer 

 tube, so that air can be purified by heating and cooling it a number of 

 times. Dr. Lodge found the dust coat to increase greatly with high 

 temperatures ; he rejects the explanations given by Tyndall, Franklaud, 

 and Lord Ealeigh ; he esi)ecially examines the effect of convecti( n cur- 

 rents in the air under high pressure, and even in water, and thinks the 

 main causes are the molecular bombardment and the gravitative set- 

 tling. His complete theory is published in the Philosophical Magazine 

 for March and April, 1884. {Nature, xxix, pp. 322, 417, Gil.) 



209. Prof. Lodge states that the concentrated radiation from an elec- 

 tric light is much less effective in warming dusty air than is the neigh- 

 borhood of a slightly warmed solid. [Nature, xxx, p. 53.) 



210. Mr. E. W. Serrell explains the flotation or buoyancy of cloud 

 particles, and also of dust, as due to adherent, heated, and dilated air, 

 namely, the sdar radiation obstructed by the particle, and consequently 

 giving rise to minute convection currents. 



211. [The pi^sent writer remembers the conversation had with Mr. 

 Serrell in 1880, to which he refers in his letter, wherein I suggested that 

 the flotation of vapor particles at upper surfttce of clouds in the atmos- 

 phere, under the influence of sunlight, was due to their evai)oration and 

 heating, by which the whole cloud is more buoyant. This, liowever, is 

 hardly parallel to the experiments made in a laboratory, where radiation 

 is of much less importance, in comparison with the condition of heat 

 by convection and kinetic molecular processes.] 



212. The observations with silk fibers, suggested by Serrell in 1880, 

 seem to iiave lately been made by Prof. G. F. Fitzgerald, of the Dublin 

 lioyal Society. {Nature, xxx, p. 331.) 



213. On the 28th of March, 1884, Prof. Osborne Eeynolds, in a lec- 

 ture at the Ivoyal Institution, summarized his results up to that time, 

 giving additional new experiments on water and air, which are 'both. 



