298 



set it on fire, and the rest did more than usual execution in the hull of 

 the vessel. Others were fired against a bank and target at 800 yards, 

 and when dug out were found, in several instances, point foremost. 

 The author concludes this paper with some observations on the 

 theory of such shells, and with remarks on their proper shape and 

 proportions, and the practical purposes to which they may be appli- 

 cable. 



On the relative Powers of various metallic Substances as Conductors of 

 Electricity. By Mr. William Snow Harris, of Plymouth, Surgeon. 

 Communicated by J. Knowles, Esq. F.R.S. November 14, 1826. 

 Read December 14, 1826. [Phil. Trans. 1827, p. 18.] 



The principle on which the author proposes to found a numerical 

 estimate of the conducting powers of metallic bodies, is, that these 

 powers are in some inverse ratio of the heat evolved during the pas- 

 sage of an electric charge through them ; and his mode of applying 

 this principle to practice, consisted in the inclosure of wires of the 

 different metallic bodies to be examined, in a given volume of air 

 contained in a glass vessel, and pressing on a column of coloured 

 liquid in a tube of small diameter communicating freely with it. 

 The heat developed in the wire by the discharge of a batter) 7 of 

 given surface charged to a given tension, being communicated to the 

 air in the globe, expands it, and raises the liquid in the tube through 

 a space, which, being read off on an attached scale, becomes a mea- 

 sure of the heat. 



After describing the precautions used to insure results comparable 

 with each other (such as those employed for obtaining an equal 

 electric discharge in each experiment from a battery of 25 square 

 feet of coated surface, the drawing of all the wires through the same 

 holes to secure this exact equality of diameter, &c.), he proceeds to 

 state the results of an extensive series of experiments. The simple 

 metals tried were copper, silver, gold, zinc, platinum, iron, tin, and 

 lead ; and the heats evolved from each were found to be in the order 

 in which they are here set down, that from copper being the least, 

 and from lead greatest, of all the substances tried, being in the pro- 

 portion of 6 to 72, or 1 to 12. The following are the principal con- 

 clusions to which his experiments have led him. 



The conducting powers of silver and copper are alike, also those 

 of iron and platinum, and those of zinc and brass. That of lead and 

 tin, he states as being in the ratio of 2 : 1, being an obvious inversion 

 of the ratio, and the same he observes of zinc and gold. Gold to 

 copper, he states as 2 : 3 ; zinc to copper or silver, as 1 : 3 ; platinum 

 or iron to copper or silver, as 1 : 5 ; tin to copper or silver, as 1 : 6 ; 

 and lead to copper or silver, as 1:12. 



The conducting powers of metals when alloyed are variously af- 

 fected. Thus, the conducting power of gold and copper, or gold and 

 silver, when alloyed together, is worse than that of either metal sepa- 

 rately, and the difference increases with the quantity of the metal of 

 inferior conducting power present. On the other hand, alloys of 



