90 BEPORT— 1882. 



Adopting -0058 as the mean conductivity of the outer crust of the 

 earth, we have 



•0058 X -000285 = 16330 x lO-'" 



as the flow of heat in a second across a square centimetre. Multiplying by 

 the number of seconds in a year, which is approximately 31^ millions, 

 we have 



1633 X 315 X 10-' = 41-4 



This then is our estimate of the average number of gramme-degrees 

 of heat that escape annually through each square centimetre of a hori- 

 zontal section of the earth's substance. 



Report of the Committee, consist! nr/ of Professor Schuster (Secre- 

 tary), Sir William Thomson, Professor H. E. Eoscoe, Professor 

 A. S. Heeschel, Captain W. de W. Abney, Mr. E. H. Scott, and 

 Dr. J. H. Gladstone, appointed for the purpose of investigating 

 the practicability of collecting and identifying Meteoric Dust, 

 and of considering the question of undertaking regular observa- 

 tions in various localities. 



1. In their first Report the Committee confined itself to giving a short 

 abstract of some of the work which had hitherto been done to clear up the 

 important question with which they are concerned. Since that time the 

 previous literature has been more thoroughly studied, and a microscopic 

 investigation of different specimens of magnetic dust derived from various 

 sources has been undertaken. 



A good deal of the literature confines itself to the dust-falls which 

 are frequently observed in the Atlantic, in the southern parts of Italy, 

 and sometimes in the Red Sea. These dust-falls were at one time sup- 

 posed to be of meteoric origin, but it has now been conclusively proved 

 that the dust has its origin in the sandy deserts of Northern Africa, 

 whence it is carried by the winds often through considerable distances ; 

 the grosser particles falling down first, so that ultimately only the finest 

 remain in suspension. With these dust-falls we are not directly con- 

 cerned, but we wish to point out that because as a whole they are proved 

 to be of terrestrial origin it does not follow that everything they contain 

 is terrestrial. Granting for a moment that meteoric dust exists, that 

 dust would accumulate in the desert as well as anywhere else, we should 

 expect that some of the magnetic particles carried hj these dust-storms 

 would show the same peculiarities which, in other cases, have led to the 

 supposition of their meteoric origin. Such indeed is the case ; but before 

 entering into details on this point we must give a short account of the 

 very clear line of argument by means of which Tissandier has to most 

 minds established the existence and general prevalence of meteoric dust. 



Tissandier has fully discussed the question in his interesting little 

 book 'Les poussieres de I'air,'' and has described the result of the micro- 

 scopic examination of the dust gradually settled down in dry weather, or 



' Paris, Gauthier Villars, 1877. 



