62. Now, if we take the kilogrammetre as our unit of work, and the heat necessary to raise a kilogramme of water 1° C. as our unit of heat, this proportion may be expressed by saying that _one heat unit is equal to 424 units of work_. This number is frequently spoken of as the mechanical equivalent of heat; and in scientific treatises it is denoted by J., the initial of Dr. Joule’s name. 63. We have now stated the exact relationship that subsists between mechanical energy and heat, and before proceeding further with proofs of the great law of conservation, we shall endeavour to make our readers acquainted with other varieties of energy, on the ground that it is necessary to penetrate the various disguises that our magician assumes before we can pretend to explain the principles that actuate him in his transformations. FOOTNOTES: [3] That is to say, a square the side of which is one centimetre, or the hundredth part of a metre. CHAPTER III. _THE FORCES AND ENERGIES OF NATURE: THE LAW OF CONSERVATION._ 64. In the last chapter we introduced our readers to two varieties of energy, one of them visible, and the other invisible or molecular; and it will now be our duty to search through the whole field of physical science for other varieties. Here it is well to bear in mind that all energy consists of two kinds, that of _position_ and that of _actual motion_, and also that this distinction holds for invisible molecular energy just as truly as it does for that which is visible. Now, energy of position implies a body in a position of advantage with respect to some force, and hence we may with propriety begin our search by investigating the various forces of nature. _Gravitation._ 65. The most general, and perhaps the most important, of these forces is _gravitation_, and the law of action of this force may be enunciated as follows:--_Every particle of the universe attracts every other particle with a force depending jointly upon the mass of the attracting and of the attracted particle, and varying inversely as the square of distance between the two._ A little explanation will make this plain. Suppose a particle or system of particles of which the mass is unity to be placed at a distance equal to unity from another particle or system of particles of which the mass is also unity--the two will attract each other. Let us agree to consider the mutual attraction between them equal to unity also.