128 ELECTRICITY 



When the thunder-cloud, thus augmented, has attained a great magnitude, 

 its lower surface is often ragged, particular parts being detached toward the 

 earth, but still connected with the rest. Sometimes the lower surface swells 

 into large protuberances, tending uniformly toward the earth ; and sometimes 

 one whole side of the cloud will have an inclination to the earth, which the 

 extremity of it will nearly touch. When the observer is under the thunder- 

 cloud after it has grown large and is well formed, it is seen to sink lower and 

 to darken prodigiously, and, at the same time, a great number of small clouds 

 are observed in rapid motion, driven about in irregular directions below it. 

 While these clouds are agitated with the most rapid motions, the rain generally 

 falls in abundance ; and if the agitation be very great, it hails. 



While the thunder-cloud is swelling and extending itself over a large tract 

 of country, the lightning is seen to dart from one part of it to another, and often 

 to illuminate its whole mass. When the cloud has acquired a sufficient ex- 

 tent, the lightning strikes between the cloud and the earth in two opposite 

 places, the path of the lightning lying through the whole body of the cloud and 

 its branches. The longer this lightning continues, the rarer does the cloud 

 grow, and the less dark in its appearance, till it breaks in different places and 

 shows a clear sky. When the thunder is thus dispersed, those parts which 

 occupy the upper regions of the atmosphere are spread thinly and equally, and 

 those that are beneath are black and thin also, but they vanish gradually with- 

 out being driven awayAp the wind. 



The instruments for electrical observation used by Beccaria never failed to 

 give indications corresponding to the successive changes in progress in. the 

 atmosphere above his observatory. The stream of fire from his conductor was 

 generally uninterrupted while the thunder-cloud was directly above it. The 

 same cloud in its passage electrified his conductor alternately with positive and 

 negative electricity. The electricity of the conductor continued to be of the 

 same kind so long as the thunder-cloud was simple and uniform in its direc- 

 tion ; but when the lightning changed its place, a change in the species of 

 electricity ensued. A sudden change of this kind would also happen after a 

 violent flash of lightning ; but the change would be gradual when the lightning 

 was moderate, and the progress of the thunder-cloud slow.* 



But among the labors of this philosopher, that rendered by modern discov- 

 eries most memorable was one which by his contemporaries and their imine.- 

 diate successors was regarded as an ingenious and over-refined conjecture, 

 rather than what it afterward proved to be, the distant shadow of a coming dis- 

 covery detected by the far-sighted rrh'nd of this acute and extraordinary man. 

 Franklin had been the first to magnetize fine sewing-needles by the electric 

 spark. Dalibard observed that the extremity of the needle at which the spark 

 from the excited glass entered had northern polarity, and both Franklin and 

 Dalibard discovered that a spark of equal force given to the other end of the 

 needle deprived it of the magnetic virtue. From these and from similar ex- 

 periments made by himself, Beccaria inferred that the polarity of the magnetic 

 needle was determined by the direction in which the electric current had 

 passed through it. He assumed the magnetic polarity acquired by ferrugin- 

 ous bodies which had been struck by lightning, as a test of the direction of the 

 electric current in passing through them, and thence inferred the species of 

 electricity with which the thunder-cloud had been charged. f 



Extending this analogy to the earth itself, Beccaria conjectured that terres- 

 trial magnetism was, like thai of the needle magnetized by Franklin and Dali- 



* Beccaria, Lcttere dell' Elettricismo. Bologna, 1758 .- p. 146, et seq. 



t " I poli del mattoue teste descritto, provano che anche in certi corpi che abbiano certa porzione \ 

 di ferro, ilfulmine imprime un scgiio jxrmanente della sua direzione." Beccaria, Lettere, p. 261. I 



