On Rain- Clouds and Atmospheric Electricity, 197 



numbers which rigorously correspond to the deductions from 

 the laws of Ohm, Joule, and Faraday. The cause of Favre's 

 result being so seriously faulty lies probably, in great part, in 

 the circumstance that, in all his calorimetric investigations, he 

 made use of the mercury calorimeter, with the use of which a 

 whole series of uncertainties are necessarily connected, and 

 which it should be a maxim not to employ. For all galvano- 

 calorimetric investigations in which the duration of the heat- 

 evolution can be chosen entirely at discretion, and so the heat 

 produced can be made as great as we please, the simple water 

 calorimeter, managed with nicety, is by far the most reliable, 

 and, for many reasons, even preferable to Bunsen's ice calori- 

 meter. The numerous measurements instituted by M. Favre 

 many years since, respecting heat-production by galvanic cur- 

 rents and electromotive forces, were very probably all vitiated 

 by an error of the same order as were the values given by him 

 for the heat developed by Daniell's and Grove's elements. 

 Should a secure basis be obtained in this department, nothing 

 remains but to repeat with more accurate methods all the more 

 important of his measurements. 



The unit of length employed in these investigations is the 

 millimetre of the cathetometer of the Physical Laboratory at 

 Zurich ; the time-unit is the second of mean time ; the Siemens 

 resistance-unit is the No. 1914, which I obtained from M. W. 

 Siemens at the commencement of the investigation, and which 

 was most carefully compared with all the other resistances 

 employed. 



Zurich, August 1877. 



XXVII. Rain- Clouds and Atmospheric Electricity. By W. E. 

 Ayrton and John Perry, Professors in the Imperial Col- 

 lege of Engineering, Tokio, Japan. 



To the Editors of the Philosophical Magazine and Journal. 



The Imperial College of Engineering, 

 GENTLEMEN, Tokio, Japan, December 8, 1877. 



/^1 IVIXG all due weight to the theories of thermoelectric 

 VX currents produced by rotation of the earth under the 

 sun, and of currents which might be produced by moving 

 electrified shells of air, we have always thought that these 

 sources of electric disturbance on the earth were far too incon- 

 siderable to give rise to the phenomena of earth-currents and 

 of atmospheric electricity, and also totally inadequate to 

 account for currents of sufficient intensity to produce ter- 

 restrial magnetism. We think that there cannot be any 



