*THE POPULAR SCIENCE MONTHLY.*

intensity of field produced by a Weber at a distance of one centimetre; and the Weber will be the absolute C. G. S. unit strength of magnetic pole. Thus the mutual force between two ideal point-poles, each of one Weber strength held at unit distance asunder, will be one dyne; that is to say, the force which, acting for a second of time on a gramme of matter, generates a velocity of one centimetre per second.

The unit of heat has hitherto been taken variously as the heat required to raise a pound of water at the freezing-point through 1° Fahr. or Cent., or, again, the heat necessary to raise a kilogramme of water 1° Cent. The inconvenience of a unit so entirely arbitrary is sufficiently apparent to justify the introduction of one based on the electro-magnetic system, viz., the heat generated in one second by the current of an Ampere flowing through the resistance of an Ohm. In absolute measure its value is 10^{7} C. G. S. units, and, assuming Joule's equivalent as 42,000,000, it is the heat necessary to raise 0·238 gramme of water 1° Cent., or, approximately, the 11000 part of the arbitrary unit of a pound of water raised 1° Fahr., and the 14000 of the kilogramme of water raised 1° Cent. Such a heat unit, if found acceptable, might with great propriety, I think, be called the Joule, after the man who has done so much to develop the dynamical theory of heat. Professor Clausius urges the advantages of the statical system of measurement for simplicity, and shows that the numerical values of the two systems can readily be compared by the introduction of a factor which he proposes to call the critical velocity; this Weber has already shown to be nearly the same as the velocity of light. It is not immediately evident how, by the introduction of a simple multiple, signifying a velocity, the statical can be changed into dynamical values, and I am indebted to my friend Sir William Thomson for an illustration which struck me as remarkably happy and convincing. Imagine a ball of conducting matter so constituted that it can at pleasure be caused to shrink. Now let it first be electrified and left insulated with any quantity of electricity on it. After that, let it be connected with the earth by an excessively fine wire or a not perfectly dry silk fiber; and let it shrink just so rapidly as to keep its potential constant, till the whole charge is carried off. The velocity with which its surface approaches its center is the electrostatic measure of the conducting power of the fiber. Thus we see how "conducting power" is, in electrostatic theory, properly measured in terms of a velocity. Weber has shown how, in electromagnetic theory, the resistance, or the reciprocal of the conducting power of a conductor, is properly measured by a velocity. The critical velocity, which measures the conducting power in electrostatic reckoning and the resistance in electromagnetic, of one and the same conductor, measures the number of electrostatic units in the electromagnetic unit of electric quantity. Without waiting for the assembling of the International Commit-