306 Electro-Meteorological Observations. 



arm should be to the perpendicular height of the kite, or that por- 

 tion of the lower surface of the positive electricity which was con- 

 centrated enough to expand the leaves of the electrometer. The 

 bag or curve of the wire was so small that it was not calculated ; a 

 small allowance might probably be made when the length exceeded 

 three hundred feet. 



The length of wire out was determined by cog wheels, (7.) with 

 a dial and hand to indicate the number of revolutions made by the 

 reel, which cog wheels were placed on the end of the axle opposite 

 to that on which the handle was placed. The height of the barom- 

 eter and dew point was also taken at the moment when the expan- 

 sion of the electrometer leaves took place. The dew point was 

 found from the following formula discovered by Mr. Espy. Take 

 two thermometers (Fahrenheit) that agree, or allow for the differ- 

 ence, cover one of the bulbs with a wet rag, and suspend them in 

 the shade where there is a draft of air, or fan them briskly until they 

 become stationary. 



Then the difference of the thermometers being multiphed by one 

 hundred and three, the product divided by the number of degrees 

 indicated by the wet bulb and the quotient subtracted from the num- 

 ber of those indicated by the dry one, will give the dew point. 



May not the aurora borealis and australis be produced by the ap- 

 proach of the positive electric fluid of the air to the surface of the earth ? 



At the poles these phenomena are frequent and brilliant ; but as 

 we approach the moist latitudes of the equator, they diminish in fre- 

 quency and brilliancy, and appear only when the dew point is very low. 



It therefore follows that it requires a very dry state of the atmos- 

 phere to permit their formation. The preceding experiments, as far 

 as they go, strengthen the opinion of Franklin,* Haref and others, 

 that there exists, at all times, in the upper regions of the air, or in 

 space beyond it, the electric fluid in an opposite state to that con- 

 tained in the earth. They also tend to establish the doctrine that 

 the positive electricity approaches the earth according to the dryness 



* " Who knows then, but there may be, as the ancients thought, a region of this 

 fire above our atmosphere, prevented b}' our air, and its own too great distance 

 for attraction, from joining our earth." — Franklin's letter to Caclicalladcr Colden, 

 April 33^, 1752. 



t " I believe myself justified in the inferences, that not only the space occupied 

 by the globe, but the region beyond our atmosphere, or where the air is sufficiently 

 rare to act as a conductor, must abound with electricity." — Prof. Hare's paper on 

 Tornadoes, Aurora, d^c. 



