-1859] WRITINGS OF JOSEPH HENRY. 257 



tender plants from the effects of an untimely frost. Though 

 the first announcement of the proposition by some of our 

 earlier meteorologists that the peculiar condition of the at- 

 mosphere known as " Indian summer " might be produced by 

 the burning of the prairies, was not thought deserving of any 

 comment, yet the advance of science in revealing the facts 

 just stated renders this hypothesis by no means unworthy 

 of attention. A large amount of smoke existing in the atmos- 

 phere must have a very sensible effect in ameliorating the 

 temperature of the season by preventing the cooling due to 

 radiation ; and although this may not be the sole cause of the 

 peculiarity of the weather above mentioned, it may be an im- 

 portant consideration in accounting for the smoky appear- 

 ance of the air and the effect produced upon the eyes. 



In concluding this section we would commend to the atten- 

 tion of the microscopists of this country, — as a readily accessi- 

 ble and interesting field of research, — the subject of atmos- 

 pheric dust. The atmosphere constantly holds in suspension 

 a mass of j^articles derived from the mineral crust of the 

 globe and from animals and vegetables, which by being 

 ■deposited in undisturbed positions, serves as a record to be 

 read by the microscope of changes alike interesting to the 

 antiquarian and the naturalist. On this subject M. Pouchet 

 has lately presented a paper to the French Academy of 

 Science, in which he enumerates the particles of mineral, 

 animal, and vegetable origin which he has found deposited 

 from the atmosphere. Under the latter he mentions specially 

 particles of wheat flour which have been found as an ingre- 

 dient of dust in tombs and vaults of churches undisturbed 

 for centuries. The dust floating in the atmosphere may 

 readily be collected by filtering the air through a tube swelled 

 in the middle, bent into the form of a syphon, partly filled 

 with water and attached at the lower end to the vent-hole 

 of a cask from which water is drawn, or simply by sucking 

 through the air by means of the mouth. 



Rain. — The discussion of the rationale of the production of 

 rain will be given in a subsequent part of this article. We 

 shall in this place however state some facts in regard to it 



17-2 



