NATURAL PHILOSOPHY. 113 



ric acid, etc. ; none of ichich are used by the magneto-electric machines 

 for generating electricity (a uniform and steady current being pro- 

 duced by Mr. Baker's last improved arrangement), thereby giving 

 them immense advantages over the galvanic batteries both in economy 

 and time. 



INCREASED EFFICACY OF LIGHTNING-CONDUCTORS. 



M. Perrot has submitted to the French Academy of Sciences some 

 interesting observations on the above subject. The metallic roof of 

 a building, he says, whether communicating with the lightning-con- 

 ductor or not, does not preserve the metallic plates below it from the 

 electric influence of the storm-cloud. Each of these plates, if it be 

 in connection with the conductor which receives the shock, sends off 

 sparks to the neighboring conducting bodies. If, then, we wish to 

 avoid accidents similar to one which recently happened to a military 

 storehouse furnished with seven conductors, it is indispensable that 

 each conductor should be placed where it will be perfectly sheltered 

 from any electric shock, a position which M. Perrot says that he has 

 obtained by a very simple arrangement exemplified in the following 

 experiments : At an explosive distance from a disc representing a 

 cloud, and in connection with the electric-machine, he places a metal- 

 lic wire connected with the earth, and representing tiie lightning- 

 conductor to be attacked. Parallel to this disc, and at a little dis- 

 tance from it, are disposed several metallic leaves or gratings, separa- 

 ted several centimetres from each other. These, designed to represent 

 the roof and the metallic plates beneath it, may be connected with 

 each other or with the conductor. Now, when these leaves are insu- 

 lated from the struck conductor, the spark and the commotion felt by 

 the hand touching these leaves are not felt very sensibly ; but if one 

 of these leaves be put in connection with the conductor it will (to the 

 exclusion of all the others) give rise to a powerful spark and commo- 

 tion. The same will occur with each leaf, and with the whole, if 

 connected with the conductor, each time that it is struck. M. Dumas, 

 who was present, said that M. Perrot had put his hand on a problem 

 of the very highest importance, and would no doubt give a complete 

 solution of it, to the great satisfaction of philosophers, architects, and 

 naval officers. 



WELDING IRON IN VACUO. 



In a communication to the Academy of Sciences, at Paris, M. Faye 

 has given the following account of some experiments in which M. 

 Ruhmkorff took part. An iron wire was cut in two, and the ends 

 brought into contact without any mechanical pressure : this done, by 

 means of an electrical current the wires were heated to a dark red 

 in vacuo, and they were thus instantaneously welded together. The 

 wires were at an angle of one hundred and fifty degrees ; and yet, 

 with that inclination, they supported a weight of upwards of three 

 kilogrammes before breaking at the point where the welding had 

 been effected. The same experiment, made in the open air, by way 

 of a counter-proof, led to no result. Encouraged by this success, M. 

 Faye heated a thick iron cylinder in vacuo, it had been sawed in 

 two, and the parts then joined together by two screws. The latter 

 10* 



