Mechanical Science. 147 



Small vessels require only one paratonnerre ; large ships should 

 have one on the main-mast and another on the mizen-mast. 



The late ingenious Mr. George Singer, in his excellent work on 

 Electricity, proposed to have conductors fixed to the surfaces of 

 masts, and the electric fluid conveyed by means of strips of metal 

 over the deck and the sides of the vessel ; but this arrangement 

 on many accounts is highly objectionable, and the mode proposed 

 by Gay-Lussac, or perhaps that commonly adopted in the British 

 navy, of conveying the electric fluid from the mast head to the 

 surface of the water, in a direct line, by means of a series of long 

 copper links, is the best which has hitherto been devised. 



It is allowed from experiment, that the stem of a paratonnerre 

 effectually defends a circle of which it is the centre, and whose 

 radius is twice its own height. According to this rule, a build- 

 ing 60 feet square, requires only a stem of 15 or 18 feet raised in 

 the centre of the roof. A building of 120 feet by the same rule, 

 would require a stem of 30 feet, and such are sometimes used ; 

 but it is better, instead of one stem of that length, to erect two 

 of 15 or 18 feet, one placed at 30 feet from one end of the build- 

 ing, the other at the same distance from the other end, and con- 

 sequently 60 feet from each other. The same rule should be fol- 

 lowed for three or any greater number of paratonnerres. A plate 

 is given in the Annals of Philosophy to illustrate this interesting 

 subject more particularly. 



15. Influenceof Coppery SfCiOn Magnetic Needles. — Nov. 22, 1884. 

 M. Arago communicated to the Academy of Sciences his experi- 

 ments relative to the oscillations of a magnetic needle surrounded 

 by different substances. He had ascertained that the copper rings 

 with which dipping needles are generally surrounded exerted on 

 the needles a very singular action, the effect of which was rapidly 

 to diminish the amplitude of the oscillations without sensibly alter- 

 ing their duration. Thus when a horizontal needle suspended in 

 a ring of wood by a thread without tension, was moved 45° from 

 its natural position, and left to itself, it made 145 oscillations be- 

 fore the amplitude was reduced to 10°. In a ring of copper, the 

 amplitude diminished so rapidly that the same needle, removed 45®, 

 from its natural position, only oscillated 33 times before the arc 

 was reduced to 10*^. In another ring of copper, of less weight, the 

 number of oscillations between the arcs of 45° and lO** were 66. 

 The time of the oscillations appeared to be the same in all the rings. 



In the ring of wood 145 oscillations from 45° to 10°. 



copper 33 45° „ 10°. 



In a lighter copper ring 66 45° „ 10°. 



16. Intensity of Electro-dynamic Force. — M. Bequerel has ascer- 

 tained, by experiments, that the intensity of the electro-dvnamic 



L 2 



