THE NATURAL HISTORY OF CLOUDS. ^Q 



also throws considerable light on the theory of rain, anci 

 other depositions from the atmosphere. By this method 

 the structure of the nimbus may at any time, when it passes 

 over us, be demonstrated to be that of a natual conductor, / 



by which the positive charge of the higher atmosphere is 

 brought down to the ^arth. F )r this purpose, there is Its ekctnea^ 

 provided a rod of iron, or other metal, well insulated on a ^'^''^ ^^^^^"' 

 pillar of varnished glass, the latter being defended from rain 

 by an inverted fupwel, soldered cr cemented to the part of 

 the rod next above it. The rod shopld be furpsshed with 

 several points of wire, a few inches long; and it need not be 

 an elevated one for this purpose, provided the eSttremity i!| 

 clear of other objects capable of drawing off the fluid. The 

 charge is ascertained by pith balls of a larger or smaller dia- 

 meter, to suit the occasion, suspended by flaxen threads, on 

 a wire fixed into the lower part of the rod, and terminating 

 in a ball. Near the latter it is proper to have .mother ball 

 fixed on a stout wire, passing into the ground, t& which the 

 fluid, when abundant, may escape in sparks. This instru- 

 ment exhibits a charge of the same kindwith that of'tbe 

 ^ir in which it is irarnersed ; or, in case of rain, &c., the 

 charge of the later, as compared with that of the air. We Phenomena 

 will give, in the first place, the appearance which we have *'^**'"^®^* 

 Recently pbserved during the passage over the rod of a nim- 

 bus of the mogt simple structure, haying neither a cumulus 

 nor a ciri-o-stratus attached to it ; w^iich rpoved along with 

 the lower current through the clear atmosphere, and dis* 

 charged a shower of large opaque hail, the air below being 

 very.vdry* During the approach of the cloud from ,the 

 north-east, the pith-balls remained close until the spreadlrtfe 

 crown, which characterizes this modification, had arrived in 

 the zenith. At this time, and while the shower itself-.vvas 

 still three or four miies distant, they opened negative. As 

 the cloud came nearer, |heir divergence increased, until it 

 amounted to full two inches, at which time sparks of con- 

 siderable gtreugth might be drawn from the rod. After 

 this the negative charge gradually went ofl^, and the balls 

 touched again. In a few moments the edge of the shower, 

 mixed with a few drops of rain, arrived at the conductor, 

 |tnd the bfxUs instantly opened positive, the charge gradually 



increasing 



