"Prof. Challis has investigated the mathematical theory of the effect of waves of condensation and rarefaction" in an elastic fluid on bodies immersed in the fluid. He concludes that the effect of such waves would be to attract the body toward the centre of agitation or to repel it from that centre, according as the wave's length is very large or very small compared with the dimensions of the body.

A tuning fork set in vibration attracts a delicately suspended body. Lord Kelvin shows that in fluid motion the average pressure is least when the average energy of motion is greatest. The wave motion is greatest near the fork and the pressure there is least, and the suspended body is therefore urged toward the fork. Maxwell remarks that all the theories which have been brought forward to explain gravitationnamely, Le Sage's corpuscle theory, the generation or absorption of fluid by bodies under pressure, the wave theory-require the expenditure of work. "According to such hypotheses we must regard the processes of Nature not as illustrations of the great principle of the conservation of energy, but as instances in which, by a nice adjustment of powerful agencies not subject to this principle, an apparent conservation of energy is maintained. Hence we are forced to conclude that the explanation of the cause of gravitation is not to be found in these three hypotheses.'

It is interesting to follow the working of Faraday's mind on the subject of gravitation. Having opened a great field in the transformations of electricity into magnetism, and of magnetism into electricity, his mind sought to embrace the force of gravitation in a generalization which should include it with those of electricity. An experiment by Faraday is always worthy of respect

ful consideration; but in following him in this excursion we feel in a certain sense like one who accompanies a voyager on a strange sea. There was nothing to guide him. No experiments, no shadowy intimations of relationship, such as had always accompanied the manifestations of electricity and magnetism. His mind naturally rested upon the electrical methods which had proved so fruitful in discovering the laws of induction, and he arranged his apparatus as follows. Since he knew nothing about gravitation except its measure and the direction in which it acted, he determined to apply the analogies of the laws of the transformation of energy, to see if some electrical work was not done when a ball of glass or wood was allowed to fall along the lines of gravitation and was immediately drawn up against the force of gravitation. It is evident that his fruitful conception of space filled with lines of magnetic and electric force occurred to him. A coil of wire moved in a certain manner across magnetic lines of force showed electrical disturbances. Such a coil, however, did not give any indications, no matter how it was moved in reference to the lines of force, of gravitation. The needle of a delicate galvanometer remained absolutely quiet if the galvanometer were connected with a coil which was moved across or along the lines of gravitating force. What led Faraday to suppose that an effect could be observed by allowing a nonmagnetic body like glass to fall through the coil and then to be drawn up through the coil against gravitation? In one case work was being done by the falling body, and in the other case work was done against the pull of gravitation. It is evident that Faraday expected the electrical equilibrium of the coil to be disturbed by this transformation in the space outside it.

Faraday, in speaking of his efforts, remarks: "The long and constant persuasion that all forces of Nature are mutually dependent, having one common origin, or rather being different manifestations of one fundamental power, has made me often think upon the possibility of establishing by experiment a connection between gravity and electricity, and so by introducing the former into the group, the chain of which, including also magnetism, chemical force, and heat, to bind so many and such varied exhibitions of force together by common relations."

"Hypothesis: Two bodies moved toward each other by the force of gravity currents of electricity might be developed either in them or in the surrounding matter in one direction; and that, as they were by extra force moved from each other against the power of gravitation, the opposite currents might be produced. A body was allowed to fall with a helix, and afterward through a helix. Various bodies were used, notably copper, bismuth, glass, sulphur, gutta-percha. It was thought that the stopping of the up-and-down motion in the line of gravity would produce contrary effects to the coming on of the motion, and that whether the stopping was sudden or gradual; also that a motion downward quicker than that which gravity could communicate would give more effect than the gravity result by itself, and that a corresponding increase in the velocity upward would be proportionally effectual. A machine was devised which could give a rapidly alternating upand-down motion.

"Here end my trials for the present. The results are negative. They do not shake my strong feelings of the existence of a relation between gravity and electricity, though they give no proof that such a relation exists."

We shall now enter upon the study of the mysteries of electricity by means of the more mysterious force of gravitation; for, as I have said, our measurements of force in general are based upon measurements of the force of gravitation. Our great unit in the study of transformations of energy is the mechanical equivalent of heat, and, as we have already pointed out, this is expressed in terms of a mass lifted a certain height against the force of gravitation.

CHAPTER III.

MAGNETISM.

To an American, the question What is electricity ? has a great national interest, for Count Rumford, Benjamin Franklin, and Joseph Henry have been like electric lights spaced on a dark path of a high mountain, and we are slowly ascending to a summit, having been guided by the rays of their genius. In America, too, the practical applications of electricity have extended with such swiftness that one might say that there is something in the manifold transformations of electricity peculiarly congenial to the American temperament. While the reader, however, may be willing to admit the claims of Benjamin Franklin and of Joseph Henry to be pioneers in the subject of electricity, he may doubt the justice of including Count Rumford with these workers on the question, What is electricity?

Our final answer to this great question will necessarily embrace the labours of Count Rumford in the subject of heat; for his celebrated experiment on the heat developed in the boring of a cannon in Munich drew mens' attention to the transformation of mechanical work into heat, and led them to reflect on the conservation of energy. Guided by this great theory, we have learned that light, heat, electricity, and magnetism can be studied and embraced under one head-that of