208 THE POPULAR SCIENCE MONTHLY. 



by the great power of the wind. The case therefore neither confirms nor 

 disproves the Espian theory, and we may still believe the well-authen- 

 ticated cases where, under favorable circumstances of very moist air 

 and absence of wind, rain has been produced by large fires." Prof. 

 Lapham also remarks, " The telegraph-wires indicated no unusual dis- 

 turbance of the electrical condition of the atmosphere." Upon reading 

 this last remark, the question occurs to us, Can there not be a change 

 in the electrical state of the atmosphere which, although too small to 

 manifest itself upon telegraph-wires, may occasion storms ? 



Some experiments made by the writer in the physical laboratory of 

 Harvard College, on the influence of flames upon the electrical state of 

 the air, may throw some light upon this subject. Two pieces of ap- 

 paratus were used, one of them " the new quadrant electrometer " of 

 Sir William Thomson, and the other a " water-dropper," also an in- 

 vention of that distinguished philosopher. The electrometer is a Aery 

 complicated piece of apparatus. Let me describe it in as clear a man- 

 ner as possible. I do not know that I shall succeed in conveying to the 

 uninitiated any idea of that instrument, for it has many parts. I shall 

 endeavor merely to explain its principles roughly. Conceive of a light 

 aluminum needle, suspended by two single cocoon threads in the centre 

 of a glass jar, which is filled to nearly one-sixth of its capacity with 

 strong sulphuric acid. A very fine platinum thread drops from the 

 aluminum needle and dips in the acid. Let us see what we have now. 

 An aluminum needle suspended in mid-air by two filaments of silk 

 very near each other, and so fine that they can hardly be perceived by 

 the naked eye. Further, this needle has an extremely fine metallic 

 wire running down from it and terminating in a little weight, also of 

 the metal platinum, which is immersed in the sulphuric acid. Thus we 

 see that the needle is very free to swing in a horizontal plane, and it 

 will be readily perceived that, if there were but one filament of silk 

 supporting it, it might swing round a complete circumference, or in- 

 deed make many revolutions under the influence of a strong repellant 

 or attractive force ; the two filaments by their torsion allow the needle 

 to swing only to a certain distance, and compel it to return to its 

 original position when the force is removed. One can readily conceive 

 of this by suspending a bar in an horizontal position by two vertical 

 ropes, and then endeavoring to turn it in an horizontal plane. 



Let us now charge the aluminum needle with positive electricity. 

 To do this, we shall conduct into the sulphuric acid by means of a 

 metallic wire a slight positive charge, and the acid, being a good con- 

 ductor, will convey this charge by the extremely fine metallic wire to 

 the aluminum needle suspended above the acid in mid-air. Now, if 

 we present a substance charged with negative electricity to the needle, 

 it will, as is well known, be speedily attracted ; and if the substance 

 presented has a positive charge it will be repelled : the unlike charges 

 attracting and the like repelling each other. Thus, we see that we 



