-1859] WRITINGS OF JOSEPH HENRY. 365 



frighten the laborers from the field. The explanation in 

 this case is too obvious to require a formal statement. 



In the equatorial regions under a vertical sun masses of 

 moist air are constantly rising during the daj'time and pro- 

 ducing electrical discharges to the earth. The vapor there- 

 fore which accompanies the reverse trade winds in the upper 

 region must be negatively electrified, while the earth in the 

 torrid zone must constantly be receiving electricity from the 

 clouds. From this we may infer that there is a current of 

 electricity through the earth, from the equator towards the 

 poles and a neutralization by means of the air above, which 

 may give rise to the aurora polaris. 



Arago has described the different forms of lightning under 

 three classes. The first class comprises the lightning which 

 consists of a vivid luminous line or furrow, very narrow and 

 sharply defined, the course of which is not a direct line, but 

 is that denominated zig-zag. This peculiar form of light- 

 ning according to Moncel is referable to the effect of partial, 

 interrupted conduction, and may be imitated by sprinkling 

 iron filings on a plate of glass ; the bifurcations of the dis- 

 charge may also be referred to the same cause. The drops 

 of rain distributed through the air perform the office of the 

 particles of iron filings in the experiment, the repulsion 

 of the electricity tending to separate it into different streams. 



The next class consists of what is called " sheet light- 

 ning," which instead of being narrowed to bright sinuous 

 lines, appears on the contrary to extend over immense sur- 

 faces. It not unfrequently has an intensely red tinge and 

 sometimes a blue or violet color predominates. The color 

 probably belongs to the flashes of lightning which take place 

 at a great elevation, and seem to illuminate lower clouds, 

 and thus to present the appearance of a broad flash. 



We may also mention that flashes of lightning are some- 

 times observed in a summer evening without thunder, and 

 known as " heat lightning," They are however merely the 

 light from discharges of electricity from an ordinary thunder- 

 cloud beneath the horizon of the observer, reflected from 

 clouds, or perhaps from the air itself, as in the case of 



