Methods /<»• Observing Atmospheric Electricity. 279 



which a portion of the earth (that is to say, a conductor connected with 

 the earth for a time) is insulated, removed from its position, and tested 

 by an electrometer, in a different position, or under cover. The first 

 method was very imperfectly carried out by Beccaria with his long 

 " exploring wire," stretched between insulating supports, on elevated 

 portions of buildings, tree tops, or other prominent positions of the 

 earth (see above, § 1); also, very imperfectly by means of " Volta's 

 lantern" — an enclosed flame, supported on the top of an insulated 

 conductor. On the other liand, it is put in practice very perfectly, by 

 means of a match, or flame burning in the open air, on the top of a 

 well-insulated conductor — a plan adopted, after Volta's suggestion by 

 many observers ; also, even more decidedly, by means of the water- 

 dropping system — described in the preceding extract— which has 

 recently occurred to the writer, and has been found by him both to bo 

 very satisfactory in its action and extremely easy and convenient m 

 practice. The principle of each of these methods of the first class may 

 be explained best by first considering the methods of the second class, 

 as follows : — If a large sheet of metal were laid on the earth in a per- 

 fectly level district, and if a circular area of the same metal were laid 

 upon it, and, after the manner of Coulomb's proof plane, were lifted by 

 an insulated handle, and removed to an electrometer within doors, a 

 measure of the earth's electrification at the time would be obtained ; 

 or if a ball, placed on the top of a conducting rod in the open air, were 

 lifted fi'om that position by an insulating support, and carried to an 

 electrometer within doors, we should also have, on precisely the same 

 principle, a measure of the earth's electrification at the time. If the 

 height of the ball in this second plan were equal to one-sixteenth of the 

 circumference of the disc used in the first plan, the electrometric indi- 

 cations would be the same, provided the diameter of the ball is small, 

 in comparison with the height to which it is raised in the air, and 

 the electi'ostatic capacity of the electrometer is small enough not to 

 take any considerable proportion of the electricity from the ball in its 

 application. The idea of experimenting by means of a disc laid flat on 

 the earth, is merely suggested for the sake of illustration, and would 

 obviously be most inconvenient in practice. On the other hand, the 

 method by a carrier ball, instead of a proof plane, is precisely the 

 method by which, on a small scale, Faraday investigated the distri- 

 bution of electricity induced on the earth's surface (see above, § 1), by 

 a piece of rubbed shellac ; and the same method, applied on a suitable 

 scale, for testing the natural electrification of the earth in the open air, 

 has given, in the hands of Delmann of Creuznach, the most accurate 

 results hitherto published in the way of electro-meteorological observa- 



