330 HISTORY OF SCIENCE. 



those of the electric spark is simply because of the larger scale of the 

 phenomena : if a spark of two inches long will pass with sharp report 

 from an electrified gun-barrel, at how great a distance will the spark 

 pass from an electrified cloud thousands of acres in extent, and how 

 loud must be the report ! 



Franklin noticed with great interest the power of pointed bodies to 

 attract electricity. He described the glow or light which is seen in 

 the dark when a pointed body is brought near a conductor, remarking 

 too that the sharper the point the greater the distance at which the 

 glow showed itself. He concludes his paper by saying : " The electric 

 fluid is attracted by points. We do not know whether this property 

 be in lightning ; but since they agree in all the particulars on which 

 we can already compare them, it is not improbable that they agree 

 likewise in this. Let the experiment be made." He describes the 

 manner in which he conceived the knowledge of the power of points 

 might be useful in preserving houses, churches, ships, etc., from light- 

 ning by fixing on the highest parts of these erections upright iron rods 

 terminating in points. The iron rods he proposes should be gilded, 

 in order to preserve them from rust, and should be connected with 

 the ground (or, in the case of ships, with the water) by a wire rope. 

 "Would not these rods," he asks, "probably draw the electric fire 

 silently from the clouds before they come near enough to strike ? and 

 by this means might not we be preserved from many sudden and 

 dreadful disasters ? ;; Franklin had intended that whenever a lofty spire 

 was erected in Philadelphia he would cause a pointed rod to be placed 

 on the ,top and insulated, so that he might ascertain whether it would 

 have the power, as he anticipated, of drawing off electricity from 

 thunder- clouds. But before he had an opportunity of putting this 

 idea into practice, the sight of a boy flying a kite suggested to him a 

 method of readily and easily raising a metallic point towards the clouds. 

 He immediately constructed a kite by stretching a piece of silk over 

 two sticks placed crosswise, and to one of these was attached a pointed 

 wire connected with the string, which was merely a piece of common 

 hemp cord. At the next approach of a thunder-storm he took his 

 kite into a field in which there was a shed, and with the assistance of 

 his son, to whom alone he had revealed his intended experiment, he 

 raised the kite (Fig. 160). The lower end of the string was held by 

 a piece of silk cord, and to the hempen string was attached a small 

 key. The experimenters placed themselves under the shed, and waited 

 in vain some time for any manifestations of electricity, although a 

 dark thunder-cloud had passed overhead. Just as Franklin was be- 

 ginning to despair of the success of his attempt, he observed that the 

 loose fibres of the hempen cord were diverging from each other. He 

 brought his knuckle near the key, when a strong spark passed. Other 

 sparks followed : a Leyden jar was charged, a shock taken from it, 

 and, in fact, all the well-known electrical experiments were performed. 



