104 ANNUAL OF SCIENTIFIC DISCOVERY. 



day in which the moisture of the air has been precipitated "by exter- 

 nal cold, and its dryness increased by its passage through the flues 

 of the furnace. In the ordinary state of the atmosphere, the elec- 

 tricity, which is evolved by friction, is dissipated as rapidly as it is 

 developed ; but in very cold weather the non-conducting or insulat- 

 ing power of the air is so much increased that the electricity, which 

 is excited by the almost constant rubbing of bodies on each other, is 

 rendered perceptible. Every person is familiar with the fact that, 

 on removing clothes or shaking garments in dry weather, the elec- 

 tricity evolved by the rubbing exhibits itself in sparks and flashes 

 of light. The popular idea in regard to this is that the atmosphere 

 at such times contains more electricity than at others; but these 

 appearances are not due to the variation of the electricity in the 

 atmosphere, but simply to the less amount of vapor which is present. 

 When the clothes are rubbed together, one part becomes positive and 

 the other negative, and in dry air the excitement increases to such 

 an intensity that the restoration of the equilibrium takes place by a 

 visible spark ; but in the case of moist air the equilibrium is silently 

 restored as soon as it is disturbed, and no excitation is perceptible. 

 Similar effects are observed on the dry plains of the western part 

 of our continent ; in rubbing the horses or mules, sparks of electri- 

 city may be drawn from every part of the body of the animal. Per- 

 sons in delicate health, whose perspiration is feebly exhaled, some- 

 times exhibit electrical excitement in a degree sufficient to surprise 

 those who are not familiar with the phenomena. But these exhi- 

 bitions have no connection with animal electricity, and are merely 

 simple illustrations of the electricity developed by friction in an at- 

 mosphere too dry to permit the usual immediate and silent restoration 

 of the electrical equilibrium. From an article entitled "Atmospheric 

 Electricity" contributed by Prof. Joseph Henry to the P\,eport of the, 

 Smithsonian Institution, 1859. 



LIGHTNING FIGURES. 



It is often stated that tree-like figures have been found on the 

 bodies of men and animals struck by the electric fluid. In 1857, 

 M. Andreas Poey, of the observatory at the Havana, brought the 

 subject under the notice of the British Association. He stated, 

 among many other cases, that in August, 1853, a little girl was stand- 

 ing at a window before which stood a young maple tree, " a complete 

 image of which was found impressed on her body after a flash of 

 lightning." The subject has been recently examined by Mr. C. 

 Tomlinson, of King's College, who has contributed a paper on it to 

 the Edinburgh Philosophical Journal. He gives an account of ex- 

 periments, during which he discharged a Leyden jar on plates of 

 window-glass previously breathed on, whereby various tree-like fig- 

 ures were produced, and in a wood-cut he exhibits one exceedingly 

 like a gnarled oak. His theory is that the impressions referred to 

 above are produced by the figures which the lightning itself assumes 

 in striking the earth, etc. M. Poey would refer their production to 

 photography, in which the lightning is the efficient agent instead of 

 the sun. M. Baudin proposes a new term for the branch of science 



