P OLYTECHNIC ASSOCIATION. 697 



ance of more compactness than that of any other class, and is seen 



only when 



From brightening fields of ether fair disclosed, 

 Child of the Sun, refulgent summer comes, 

 And Cancer reddens with the solar blaze. 



This is the principal laboratory of thunder, and hence the lightning 

 of our summers most commonly proceeds. Of the vast extent of this 

 laboratory we may form some conception, if we but estimate the 

 height or depth, with the length and breadth, of these wide-spread 

 masses and their innumerable circumvolutions, covering and moving 

 over hundreds and thousands of square miles of the earth's 

 surface. They are distinguished from other clouds by their form 

 and color, but their chief characteristic is their wonderful accumula- 

 tion of electricity. We cannot be far wrong in ascribing to the 

 excess of this fluid the peculiar shapes assumed by those clouds, and 

 from their infinite variety we may be sure that all the extraordinary 

 variations of the earth's surface are reproduced and multiplied in those 

 changing masses ; there are eminences with round and pointed sum- 

 mits, promontories, bluffs, perpendicular walls, coves, caverns, plains, 

 valleys, steep declivities, gradual slopes, lozenge and lens-shaped sur- 

 faces, convex and concave — in short, every various form that the 

 imagination can conceive. Considering that the sun, flaming down 

 upon these multiform surfaces, strikes some with rays direct, while 

 his rays glance in every degree of obliquity upon others, we can com- 

 prehend how different portions of this mighty field of electrical opera- 

 tion may be variously affected, and how the accumulation of the fluid 

 may be very unequally distributed. We can understand how a focus 

 may sometimes be produced by means of a lens-shaped cloud, concen- 

 trating the sun's rays and causing intense heat. Among the many 

 sources of electricity, the combined operation of heat and moisture 

 and sudden change of temperature are known to be the most poten- 

 tial and prolific. These causes are present and acting together in the 

 thunder cloud of our summers, which accordingly exhibit, beyond all 

 comparison, the most magnificent displays of electrical action and the 

 most abundant development of disengaged and current electricity 

 that are ever witnessed. The various ingenious instruments and 

 electrical machines, contrived for the purpose of developing electricity, 

 present but minute imitations of these grand exhibitions of nature. 

 In contemplating such splendid phenomena, the mind naturally turns 

 to inquire whence those immeasurable quantities of the electrical 

 fluid are derived. 



