45, COENHILL, E.G., AND 122, EEGENT STEEET, W., LONDON. 119 



172. Collection of Electricity. "A simple rough method of doing this 

 is to shoot a metallic arrow upwards into the air, the arrow being tied to one 

 end of a conducting string, the lower end of which carries a ring which rests 

 upou the electroscope. The arrow being shot upwards, the electroscope will be 

 found to be electrified, as it mounts ; and when the ring leaves the plate, the 

 instrument will indicate the state of electrification of the air at that point where 

 the arrow is at the time. 



"This manner of observing is simplified by substituting a long conductor 

 reaching upwards; a gilded fishing rod may be employed, its lower extremity 

 being insulated. 



" The usual method employed, however, is Volta's, in which the electricity 

 is collected by means of a flame, burning at a height, either in a lantern hung 

 to mast, and connected to the electroscope by a wire, or, by a slow burning 

 match attached to the top of a long metal rod. 



" The electricity of the air in the neighbourhood of the flame, by its induc- 

 tive action upon the conductor, causes electricity of the opposite nature to 

 accumulate at the upper extremity, where it is constantly carried off by the 

 convection currents in the flame, leaving the conductor charged with electricity 

 of the same kind and potential as the air."* 



173. " The princple of Volta's method has been made use of by Sir W. 

 Thomson in his Water-dropping Collector, now employed in observatories, and 

 found to be extremely useful for the observation of atmospheric electricity. 



A copper can is placed on an insulating support, which may be of ebonite, 

 having the surface thinly coated with paraffin ; or of glass surrounded with pumice 

 stone soaked in sulphuric acid. From the can a small pipe projects a consider- 

 able distance into the air, and terminates in a fine orifice. The can being filled 

 with water, and the tap which opens into the jet pipe turned on, a small stream 

 of water is allowed to flow out, care being taken that it is so small that it shall 

 break into drops immediately after leaving the nozzle of the tube. 



In half a minute from the starting of the stream the can will be found to 

 be electrified to the same potential as the air at the point of the tube. 



This Collector cannot be employed during the time of frost, unless means 

 are adopted to prevent the freezing of the water in the jet pipe. When obser- 

 vations are to be made with a portable instrument, a slow burning match 

 should be used. Sir William Thomson recommends for this purpose blotting 

 paper, steeped in a solution of nitrate of lead, dried, and rolled into matches. 



As to the position of the Collector, since electrical density is greater on 

 projecting surfaces, and less on hollow surfaces than on planes, the Collector 

 should not be near trees or houses, nor within a closed space."* 



The above Apparatus and Collecting Match made to order. 



* Robert H. Scott, Esq., Meteorological Office. 



