^6 ANNUAL RECORD OF SCIENCE AND INDUSTRY. 



can not always be the same. In short, the electric dis- 

 charges which take place in the polar regions between the 

 positive electricity of the atmosphere and the negative elec- 

 tricity of the terrestrial globe are the essential and only 

 causes of the formation of the aurora, whose existence is, in- 

 deed, independent of that of terrestrial magnetism, since the 

 latter only im^^arts to it a certain direction, and in some cases 

 a movement. During auroras the most numerous and the 

 most intense currents of terrestrial magnetism flow from the 

 north to the south ; but occasionally they are observed, flow- 

 ing from the south to the north. These latter, according to 

 Lemstrom, are currents of induction ; an opinion in which 

 De la Rive agrees. Report of Sec. of jS?nithso7iicm Listitu- 

 lion for 1874, 234. 



ON THE POWER OF LEAVES TO ABSORB AND RADIATE HEAT. 



The power of leaves to radiate heat has been investigated 

 by Maqueune, Avho has for this purpose employed the Leslie 

 cube, one of whose faces was blackened and the other cov- 

 ered with the leaves to be studied. The temperature of the 

 water in the cube did not exceed 40 Centigrade, so that the 

 mechanical structure of the leaves was unaltered. Of eight 

 plants whose leaves were examined, the radiative powers av- 

 eraged from ninety-one to ninety-seven, that of the blackened 

 face of the cube being one hundred. And it may be assumed 

 that the radiative power is on the average ninety-five for all 

 of these leaves, and possibly for most others. On the other 

 hand, the absorptive powers of the leaves were observed by 

 covering one of the two faces of a thermopile with lamp- 

 black, the other with the leaf in question, and exposing these 

 successively to a warm piece of blackened wood. The ab- 

 sorptive powers of these leaves varied, as before, from ninety- 

 four to ninety-seven, the average being ninety-five; and it 

 was concluded that for obscure heat, (1) the emissive and 

 absorptive powers were sensibly equal ; (2) they are nearly 

 equal to that of lampblack; and, finally, the determination of 

 the quantity of dew which is deposited upon plants ought to 

 be made by means of blackened pluviometers, or with gauges 

 covered with some substance having a very considerable ra- 

 diative power. If this were done, we probably should be 

 able to explain the discrepancy which exists when we com- 



