130 ANNUAL OF SCIENTIFIC DISCOVERY. 



to judge, I infer that heavy velvet carpets answer this purpose best. 

 Two thicknesses of ingrain carpeting answer very well. A drugget 

 spread upon an ingrain carpet yields a good supply of the fluid. The 

 effect of the increased thickness is obviously to improve the insulation 

 of the carpet. The carpet must be quite dry, and also the floor of 

 the room, so that the fluid may not be conveyed away as soon as it is 

 excited. This will not generally be the case, except in winter, and in 

 rooms which are habitually kept quite warm. In warm weather, only 

 feeble signs of electricity are obtained. The rubber, that is, the shoe, 

 must also be dry, and it must be rubbed upon the carpet somewhat 

 vigorously." In instances where persons are said to have drawn 

 electric sparks from a coal stove and a common grate, Prof. Loomis 

 considers it probable that the experimenter, and not the stove or grate, 

 was the electrified body. 



PROPER LENGTH OF LIGHTNING CONDUCTORS. 



THE rule prescribed by the French Academy is, that a lightning-rod 

 will protect a circle whose radius is twice the height of the rod ; but 

 Prof. Loomis cited to the American Association at New Haven an in- 

 stance which, he says, " demonstrates to my mind that it is unsafe to 

 rely upon a rod to protect a circle of a radius larger than one and a half 

 times the height of the rod, at least upon the west side, whence most 

 of our thunder-showers come." These observations drew out various 

 remarks. Prof. Henry stated that he had found in trees struck by 

 lightning that there would be no traces of electricity on the upper 

 branches, but it appeared to strike at the main trunk. He had ob- 

 served that, when the color of the electric discharge is red, it indicates 

 that the electricity is very high. 



USE OF GUTTA-PERCHA AS MEANS OF ELECTRICAL EXCITATION. 



MR. W. H. BARLOW describes in Brewster's Philosophical Magazine 

 for December a new electrical machine. It consists of a wooden frame 

 carrying two wooden rollers, the lower one being about six inches in 

 diameter, and having a handle attached to the axle. The upper roller 

 is about half as large. A band of thin sheet gutta-percha, about four 

 inches wide, is made to pass round the rollers, fitting them very tightly. 

 Near the upper roller are two cushions covered with silk, and con- 

 nected together so as to press upon opposite sides of the gutta-percha 

 at their upper extremities, and opening towards their lower extremities 

 at an angle of about twenty degrees. When the handle of the ma- 

 chine is turned, causing the gutta-percha band to pass over the rollers 

 at a moderate velocity, electricity is given off about three or four inches 

 below the cushions, and by applying a conductor the apparatus may 

 be used as a common electrical machine. The quantity of electricity 

 developed increases with the surface of gutta-percha. Gutta-percha 

 may be excited both positively and negatively. If a strip about two 

 feet long and two inches wide be laid on a surface and rubbed, the two 

 extremities when suspended in the air repel each other, and the elec- 



