206 APPENDIX. [No. XII. 



the air and earth exceedingly dry; but occasionally the lightning- cloud 

 is formed when the surface of the earth is saturated with wet, and the 

 air highly hygrometric. In this latter case, a dense cloud is generally seen 

 to form in the atmosphere without any apparent cause ; with the utmost 

 rapidity, and within a few minutes, and sometimes within a few seconds 

 of its appearance, hail, rain, and lightning follow. 



"The lightning-cloud sometimes expands itself nearly at the place 

 where it is first formed ; and in this case, perhaps, the cloud and earth are 

 in an uniformly opposite state, which would have the effect, by virtue of 

 the attraction of oppositely electrified surfaces, of causing the cloud to be 

 retained in its position. Sometimes, however, without any wind, the cloud 

 takes a rapid travelling fit, crossing England in a few minutes, and paying 

 our French neighbours a visit, as we find by the account in the papers 

 the next day, striking and carrying devastation in its progress. Some- 

 times the cloud will travel away for two or three hours, and then travel 

 back; sometimes it will take a circular motion, and, in fact, the freaks 

 which a travelling cloud will pay are innumerable. The travelling cloud 

 may possibly owe its properties to an unequal tension at different parts ; 

 for if one end of the cloud, or the entire cloud, had an attraction to the 

 earth under this end, and in advance of it, the cloud would be drawn 

 towards that part ; but as there is a force which resists its direct down- 

 ward attraction to the earth, it moves in the diagonal of the force tending 

 to raise the cloud, and the force drawing all the cloud to one point, when 

 a motion more or less rapid must be the result. In this case we must 

 suppose that the earth under one end of the cloud as it advances imme- 

 diately assumes a violent tension." P. 102. 



The subjects next considered are the effects of the tensive electricity 

 exhibited by the thermo-electric and magneto-electric apparatus, and by 

 the hydro -electric apparatus, in which the force is generated apparently 

 by the friction of steam, a gigantic specimen of which is exhibited at the 

 Royal Polytechnic Institution. The remainder of the chapter is devoted to 

 dynamics force, its origin, and various modes of adaptation to the wants 

 of mankind. But this we pass over, in order to arrive at Chapter IV., on 

 the sciences of actions and reactions. 



Action is the exertion of new attractions ; reaction, the tendency to 

 the maintenance of old ones. When these two forces are nearly balanced, 

 vibrations ensue. Here let us show our author's idea of the nature of time, 

 and its essential dependence on matter. He observes, first of all, how 

 fortunate it is for the economy of the world that attractions meet with 

 opposition. 



" Former attractions act as an impediment to the exertions of the new 

 ones ; the energy of the desire for combustion, of carbon for oxygen in our 

 fires, is held at bay by the former attraction of the particles of coal, which 

 is gradually and progressively overcome. Our fires, therefore, burn 

 regularly and steadily, our candles with slowness and precision, and all 

 other actions, even to the railway engine, take place with an energy pro- 

 portionate to the resistance of the new action which causes the effect. 



" The energy with which a new attraction overcomes an old one is 

 called the time of its performance; and, conversely, the energy of the 

 resistance to a new action by an old one is called the time at the attempt 

 of performance. Time, therefore, is the abstract idea of the energy of an 



