August 29, 1884. 



SCIENCE. 



165 



and has also adopted Thomson's water-drop- 

 per (which consists merely of an insulated can 

 of water connected with the quadrants of the 

 electrometer, the water issuing from this can 

 in small drops, reducing it to the potential of 

 the air) , still there are those who believe that 

 this method does not give correct res-ults. 

 Professor Palmieri, who has been connected so 

 long with the meteorological stations on Mount 

 Vesuvius, rejects Thomson's electrometer, and 

 the water-dropper also. He believes that the 

 electricity of the air is not led to the water- 

 dropper by conduction, and that the insulated 

 water-can does not take the electricity of the 

 air by any similar process. According to his 

 views, the electrical state of the air can be as- 

 certained by its inductive effect upon a disk of 

 metal which is suddenly elevated or lowered in 

 the air ; and he has devised a special electrom- 

 eter, strongly resembling Peltier's electrome- 

 ter, and a special apparatus for elevating and 

 lowering a disk. He, moreover, does not 

 think that continuous photographic registra- 

 tions are of much use, since electrical obser- 

 vations are only of value when accompanied 

 by observations on the condition of the sky 

 with respect to clouds, and upon the direction 

 of the wind. 



Professor Palmieri' s methods and instru- 

 ments do not impress us very favorably. There 

 must be great difficulty in insuring good insula- 

 tion by his method of suddenly elevating a disk 

 in the air. Moreover, his theory of induction 

 does not appear to us to be well founded. The 

 Thomson method of observing atmospheric 

 electricity seems to promise better results than 

 any other ; yet it is not by any means perfect, 

 especially in its practical adaptation to the 

 needs of a government signal-service. The ex- 

 periments which are being conducted at the 

 physical laboratory of Harvard college show 

 that in the American climate it is extremely 

 difficult to secure a regular flow of water from 

 the water-dropper, and to obtain good insula- 

 tion, on account of frost and snow. During 

 the months which are free from snow and ice, 

 heavy showers wet the insulating stand of the 

 water-dropper, and thus destroy the insulation. 

 The latter evil can be obviated to some degree 

 by a well-constructed screen of wood. It has 

 been found preferable to neglect the insulation 

 of the can, and allow the drops of water to 

 fall upon a metallic plate, thrust out from the 

 side of the room in which the electrometer is 

 situated, by means of a glass cylinder through 

 the centre of which runs a wire which connects 

 the metallic plate with the electrometer. The 

 drops of water fall, in turn, from the metallic 



plate, and reduce this to the potential of the 

 air, while the insulation of the metallic disk 

 can be perfectly maintained in all weathers. 

 Preliminary experiments have also been made 

 upon an arrangement which promises to be of 

 use in winter, when the weather would prevent 

 the use of the water-dropper. This arrange- 

 ment consists merely of a wheel provided with 

 metallic brushes. The wheel is run rapidly by 

 simple clock-work, and is insulated. The 

 brushes touch one end of an insulated con- 

 ductor exposed to the air, and then touch a 

 conductor connected with the earth, in this 

 way imitating the action of the water-dropper. 

 An arrangement of this kind, which will work 

 in all changes of weather, is essential in the 

 climate of the United States. 



The preliminary experiments at the labora- 

 tory of Harvard college have also shown that 

 it is essential that the electrometer should not 

 be very distant from the water-dropper or its 

 equivalent. A naked iron wire connected the 

 electrometer with a water-dropper which was 

 about three hundred feet distant at the top 

 of a building, and at least sixty feet from the 

 ground. The photographic record of the ex- 

 cursions of the electrometer needle showed 

 that it moved irregularly to and fro under the 

 influence of the fluctuation of potential along 

 the wire. There is evidently a certain relation 

 between the size of the conductor, which is 

 reduced to the potential of the air by the suc- 

 cession of water-drops, and the number of ori- 

 fices from which the water must issue in order 

 to reduce the conductor and connecting wire to 

 the potential of the air. 



The photographic records that have been 

 made show unmistakably that north-west winds 

 in the colder months are preceded by a rise 

 in the electrical potential of the air, and that 

 during an east wind the potential falls. These 

 general indications seem to be independent of 

 looal effects, and lead us to believe that elec- 

 trical signal-station observations will be useful 

 in predicting changes of weather. Photographs 

 of the varying electrical state of the air could 

 be forwarded to Washington from different 

 stations, and a map could be made on which 

 stations at the same electrical potential could 

 be connected ; and thus any law connecting 

 the electrical state of the atmosphere with 

 other meteorological changes could probably 

 be ascertained. Much remains to be done, 

 however, in ascertaining the best position for 

 such signal-stations, and in perfecting simple 

 and practical apparatus for the use of com- 

 paratively unskilled observers. 



John Tkowbridge. 



