22 PROCEEDINGS OP THE AMERICAN ACADEM5T 



an outward breeze is produced. There is always a breeze from an 

 area where the air is condensed, and towards an area where it is 

 rarefied. This local condensation and rarefaction of the air are the 

 cause of breezes and winds and hurricanes. The maps issued by 

 our weather bureau show areas of high and areas of low barometer, 

 and the direction of the wind is always from the one to the other. 



If, instead of allowing the wind to blow freely through space, we 

 confine it to a narrow channel, — if, for instance, we produce an air 

 pressure at one end of a long iron pipe, we get through the pipe a 

 strong current of air, — as, for example, in pneumatic tubes, in 

 which the current of air carries along small carriages, in the tin 

 pipes used to convey the hot-air currents from our furnaces to the 

 registers, and in the pipes used for distributing illuminating gas 

 from the central gasometer to the houses about the city where it is 

 burned. 



The wind blowing through space is made evident to our eyes 

 by its scattering of a handful of dry leaves, or dust, or bits of 

 paper. It bows and bends the branches of the trees. A candle 

 flame exposed to it is distorted, or instantly blown out. Its direc- 

 tion is more accurately indicated by the familiar weather-vane, and 

 its strength is measured, if we will, by the anemometer. 



As with the wind of nature, so too with the electric wind. If 

 we substitute for our Pacific island a metal ball suspended in mid- 

 air by a silken cord, and let the ocean be represented by the sur- 

 rounding atmosphere on a moist day, and produce upon the ball an 

 area of electric condensation, or, as an electrician would say, a posi- 

 tive electric change, we shall have an electric breeze blowing out- 

 ward in all directions, which will last so long as the electrification 

 of the ball is above that of the surrounding air. This may be made 

 evident to the eye by placing upon the ball a handful of bits of 

 paper, or dry leaves, or dust, any of which will be instantly blown 

 away and dissipated in all directions. If a lighted candle be 

 brought near the ball, its flame will be bent by the breeze, and 

 possibly be blown out. 



If instead of allowing this electric wind to blow away in all 

 directions we confine it to a narrow path, — if, for instance, we con- 

 nect the ball to the earth by a metallic wire, — we get through 

 the wire a strong current of electricity, whose direction may be 

 determined by the electric weather wave, a magnetic needle, and 

 whose strength may be measured, not with the anemometer, but 

 by the galvanometer. 



