B. TERRESTRIAL PHYSICS AND METEOROLOGY. 115 



distribution thereof is not necessarily invariable. It would 

 naturally be greater at the summits of mountains and less in 

 the midst of perfect plains. This theory, although very wide- 

 ly applauded, and in very many respects satisfactory, does 

 not explain the diminution of electricity at the poles as com- 

 pared with that at the equator. Miihry has, therefore, lately 

 broached a new hypothesis calculated to remove this diffi- 

 culty. He considers the Avarming of the earth's surface by 

 the sun as the source of atmospheric electricity ; that, there- 

 fore, the distribution of the quantity of electricity increases 

 and diminishes with the distribution of temperature, not only 

 from the equator to the poles, but also from hour to hour 

 throughout the day and the year. Miihry gives us, however, 

 no idea as to the method by which the sun's heat is con- 

 verted into electricity, nor does it seem at all certain that 

 such a hypothesis as he has empirically instituted, however 

 well it may explain certain phenomena, is allowable, unless 

 further data can be adduced than he has o-iven to render it 

 probable. Equally in consonance is it with the present con- 

 dition of our knowledge to think that, while in reality the 

 intensity of the atmospheric electricity does increase parallel 

 with that of the sun's heat, this fact is due i-ather to the 

 other parallel circumstance, namely, that the mass of aqueous 

 vapor in the atmosphere increases with the sun's altitude ; 

 and, as it rises into the atmosphere, there results, at least in 

 continental climates, in the middle of the day, in the lower 

 part of the earth's atmosphere, a diminution not only of the 

 moisture, but also a diminution of the quantity of electricity. 

 In some such way as this it would follow that the daily va- 

 riation of the electricity is synchronous with the daily varia- 

 tion in temperature, in moisture, and in barometric pressure. 

 It is very desirable, for the further test of Miihry's hypothesis, 

 that there should be made accurate observations upon this 

 subject on the higher mountain peaks, where the daily change 

 of the electricity as compared with the state of the atmos- 

 phere should be more accurately determined. 7 (7, 1873, 322. 



THE HAIR HYGROMETER VeVSUS THE PSYCHROMETER. 



Professor Wolff, of Zurich, in a recent number of the Me- 

 teorology of jSwitze7'land,has given the results of some studies 

 upon the much-vexed question as to the reliability of the 



