POPULAR MISCELLANY. 



573 



it. It is probably a natural form of Leyden 

 jar very highly charged. The light of a 

 tiash, Professor Tait states, is of very much 

 greater intensity than is commonly sup- 

 posed, the apparent brightness being, on 

 account of the exceeding small duration, 

 less than one hundred thousandth part 

 of what it would be if the lightning were 

 permanent. This duration is not more 

 than the millionth part of a second, and 

 hence it is a mistake to suppose that you 

 can sec the direction in which the discharge 

 is passing, whether from the clouds to the 

 earth or from the earth to the clouds. Pro- 

 fessor Tait insists upon the necessity of 

 properly pointing and grounding lightning- 

 rods to secure safety, and mentions several 

 instances of the ignorance in this matter 

 which seems to prevail among even well- 

 educated people. In one case the point of 

 the rod was actually covered with a glass 

 insulator. The best ground connection is 

 with the water-mains, but large masses of 

 metal or other conductor in a moist soil an- 

 swer when these are not at hand. Connec- 

 tion with a body of water inclosed in a ma- 

 sonry basin, such as a reservoir, is not a 

 proper way of grounding the rod. A light- 

 ning-rod acts as a constant drain upon the 

 charge in the clouds in its neighborhood, 

 and when the rods are numerous over any 

 area, as in a well-protected town, storm- 

 clouds will pass over without any lightning- 

 flashes taking place, though such form of 

 discharge will occur before reaching the 

 protected district and after passing it. The 

 lecturer insisted that people should regard 

 the use of rods as both a public and private 

 duty. In regard to the sources of electrici- 

 ty in the atmosphere. Professor Tait has 

 been led by experimental researches to in- 

 fer that a separation of the opposite elec- 

 tricities occurs simply by the contact of 

 particles of air and aqueous vapor, which, 

 on the kinetic theory of gases, are in con- 

 stant collision. In the same way zinc and 

 copper become oppositely electrified when 

 brought into contact. The precipitation of 

 vapor-particles into cloud-particles and the 

 agglomeration of these into rain-drops enor- 

 mously increase the electrification, as the 

 potential of a free charged sphere is pro- 

 portional directly to the quantity of elec- 

 tricity on it and inversely to its radius. The 



separation of these highly charged particles 

 of air and water is effected by gravity and 

 the diffusion of gases which would cause 

 the air-particles to escape from among the 

 mass of precipitated vapor to the less highly 

 electrified air above. Sir William Thomson 

 has offered an explanation of the phenom- 

 ena, based upon the fact that the lower air 

 is usually negatively charged. Ascending 

 currents carrying this air upward, the elec- 

 tricity, which was formerly spread out over 

 a large area, may by convection become so 

 much less diffused that it will be raised 

 to a high enough potential to give a spark. 

 However the electrification of the precipi- 

 tated vapor occurs, there is no question 

 about the fact that, clouds once formed, the 

 particles are electrified. The solution of 

 the problem of just how this is brought about 

 must in all probability come through exper- 

 iments made on a larger scale than any so 

 far conducted in the laboratory. 



Science in the Schools of France. The 



modifications in the course of studies in the 

 French public schools, recently decreed by 

 the Superior Council, give to scientific teach- 

 ing a more prominent place than has hither- 

 to been allowed it, especially in the element- 

 ary classes. In the seventh class, the ele- 

 ments of the natural history of animals and 

 plants are added to the history of soils and 

 stones, and take the preference over it, as 

 offering more interest to children and being 

 of greater practical utility. In the sixth 

 class, an hour is deducted from the ten de- 

 voted to Latin and added to those given to 

 the sciences, which are allowed four hours 

 a week. In the fifth class, where scientific 

 instruction has been obviously deficient, the 

 hours for Latin are reduced to five, and 

 the sciences are given four hours. In the 

 fourth class an hour is taken from Greek, 

 and the hours for scientific instruction are 

 increased to four. Scientific instruction will 

 be continued in the third, second, and rhe- 

 torical classes without encroaching upon the 

 other courses, an hour being taken from the 

 study-hours for new subjects of natural his- 

 tory in the third class, for physics in the 

 second class, and for subjects of physics 

 which have not been previously entered 

 upon by the pupils, in the rhetorical class. 

 I The Superior Council advises that the teach- 



