then he lived before Mr. Gladstone's cheap wines, and escaped divers aches of which you know but too well. Suppose you called him from the grave, and asked him if he had caught "neuralgia" from sleeping so long in the wet ground, his fleshless jaws would laugh in your face and say he knew not what you meant. As for heart diseases and kidney diseases the doctors had not as yet found them out. Of the spleen he knew something, but then he thought it came from the climate, and that "Port" was "sovran" for it. In these days the doctors call it dyspepsia and liver, and now we look at you we think that old disease is the one you have got, and if you do not take care it will turn this summer to kidney or heart or head disease. But the plain truth is you are too happy and comfortable at home, your wife is too good to you, your children are too fond of you; in society we remark that you are long-winded; at the club people begin to vote you a bore. You subscribed too to the "Metropolitan Memorial to Shakspere," that looks very much. like softening of the brain. For Heaven's sake don't tempt Providence any longer. Don't stay here where people look up to you and respect you--for your money, but fly to some land where you must learn to shift for yourself, cease to eat your food alone, learn also to kill it. If needs be wash your own shirts. Then you will respect yourself, which you cannot do now, when every one has heard the truth of you from us, and then you will be able to bear the respect of others. Follow therefore, dyspeptic brother, the example in the flesh which we have set you in the spirit. Fly from your wife and family. Have a thorough outing, make yourself as uncomfortable as you can, and when you come back with renewed strength and spirits thank us for having shown you the way to Faroe. ART. III.-1. JOULE. Series of Papers in the Philosophical Magazine in 1841 and subsequent years. 2. MAYER. Bemerkungen über die Kräfte der unbelebten Natur. Liebig's Annalen, 1842. Die organische Bewegung in ihrem Zusammenhange mit dem Stoffwechsel. Heilbronn, 1845. Beiträge zur Dynamik des Himmels. Heilbronn, 1848. 3. HELMHOLTZ. Ueber die Erhaltung der Kraft. Berlin, 1847. Lectures on the Natural Law of Conservation of Energy, delivered at the Royal Institution. Medical Times and Gazette, April 1864. 4. Exposé de la Théorie Mécanique de la Chaleur. Par M. VERDET. Paris, 1863. In our recent article on The Dynamical Theory of Heat we considered at some length the absurdity of attempting to base extensions of Natural Philosophy upon mere metaphysical speculations; and we showed that without direct experimental proof, or the less direct but still conclusive proof furnished by rigorous mathematical deductions from experimental results, nothing can with any show of reason be predicated of the laws of Nature. Experience is our only guide in these investigations, for there can evidently be no à priori reason whatever why matter should be subject to one set of laws rather than another, so long at least as each of these codes is consistent with itself. We particularly instanced the caloric or material theory of heat, as not only unjustifiable in itself, but (while it was received) antagonistic to all real progress. The corpuscular, or material, theory of light furnishes another excellent example. The preposterous nonsense that was gravely enunciated, and greedily accepted, with regard to the nature and laws of light, and the elaborately absurd properties assigned to its supposed particles in order to fit them for their everyday work, would be almost inconceivable to a modern reader, were it not that equally, or more, extravagant dicta of the "great inexperienced" have been, and are even now, propounded by self-constituted interpreters of the original designs of Nature. And we nowhere find them more prevalent, or more pernicious, than in the case of the grand question which we are about to discuss. We have no more reason, before experiment settles the question, to fancy Energy indestructible than the Calorists had for believing in the materiality of heat. The philosophers who said that " Nature abhors a vacuum" had at least an experimental basis for their guidance; and, if they had limited the generality of their statement to the class of circumstances really involved in their experiments, we might have smiled at the peculiarity of the language in which their conclusion was expressed, but we must have allowed it to be correct. But when we find, in modern times, a sermon, however able, founded without experiment on such a text as" Causa æquat effectum," we feel that the writer and his supporters are little in advance of the science of the dark ages, and are irresistibly reminded of the famous Tenterden Steeple. This is the fundamental characteristic of all the writings of Mayer, and therefore we may for the present leave them unnoticed, although we shall afterwards have occasion to consider them as furnishing a most admirable development of the consequences of an unwarrantable assumption. For there can be no more doubt that the works of Mayer, above enumerated, contain highly original and profound deductions from his premises, than that those premises were unjustified by experiment, and therefore not only unphilosophic but destructive of true scientific method. Let it not be imagined that we undervalue the assistance which science often receives from the wildest speculations-so long as these are not elaborately enunciated as à priori laws, but are confined to their only legitimate use, the suggestion of new methods of interrogating nature by experiment. By all means let philosophic minds indulge in any vagaries they may choose to foster, but let these be kept as private magazines from which, when required, may be extracted an idea leading to an experimental research. In perhaps one case in a million, the expected result may follow: but, in the many cases in which it does not occur, there are thousands of chances (which will not be lost to the careful experimenter) of discovering something utterly unlooked for. We might give instances of this without number. The discovery of electro-magnetism by Erstedt was arrived at by his fancy that a conducting wire might act on a magnet if heated by an electric current. Kepler's Laws were deduced by an almost incredible amount of numerical calculation based upon the supposition of the existence of all sorts of harmonies, perfect solids, etc. etc., in the solar system. In chemistry this principle has been long recognised as most important, since, in the attempt to produce directly some particular compound, it often happens that the experimenter is gratified by the appearance of some other which he had never dreamt of as capable of existing, or at least of being obtainable by his process. Mayer, therefore, and others who have followed a course similar to his, cannot be considered as having any claims to the credit of founding the science of Energy; though their works have become of great value as developments and applications, since the science has been based upon rigorous experiments. Potential and Kinetic Energy. 339 Particular cases of the Conservation of Energy were experimentally discovered, but without any reference to this principle, at early stages of the progress of electricity, electro-chemistry, heat of combination, and various other branches of science; and many curious cases of Transformation and Dissipation of Energy had also been observed. To these we shall advert after we have given a brief sketch of the Laws of Energy and the history of their discovery; as we shall then be enabled to classify them properly, and to show their mutual connexion. In order that we may understand clearly the terms which it is essential to employ in giving a strictly accurate, although popular, view of these great Laws, it will be useful to give preliminary examples of various forms of energy constantly presenting themselves to our notice. Let us take, for instance, gunpowder. It contains in a dormant form an immense store of energy, or, in common mechanical language, it can do an immense amount of work. Its use in blasting is simply to do at little expense, and in a short time, an amount of work which it would take many labourers a considerable time to perform. In virtue of the arrangement of its chemical constituents, it possesses this store of work-producing power. Again, in order that water in a reservoir may be capable of supplying motive power to mills or other machinery, it must be capable of descending from a higher to a lower level, for no work can be got out of still water, unless it have a head as it is technically called. When the driving-weight of a clock has run down, the' clock stops; and in order that the weight may be again efficient in maintaining the motion of the wheels and pendulum, it must be wound up, or placed in such a position relatively to the earth, that work can be got out of it in consequence of its position. In an air-gun we have a store of energy laid up in the form of compressed air; in a cross-bow, a wound-up watch, or the lock of a cocked gun-in the form of a bent spring; in a charged Leyden jar-in the form of a distribution of electricity; in a voltaic battery-in the arrangement of chemical elements or compounds; in a labourer, primed for work-in the form of a proper supply of food. In all such cases, where the energy is dormant, it is called Potential Energy; and its amount is measured by the work which it is capable of doing, and which it will do if properly applied. It would be easy, but unscientific, to break out into thrilling descriptions of the terrors of the impending avalanche, the dangers of the slippery precipice, etc. etc., all of which are mere cases of potential energy; to paint the agonies of the wretch transported to such a planet as Jupiter, where his potential energy, when standing upright, would be for ever increased, as if he carried other two men on his shoulders; or his Atlas-like position if taken to the sun, where he would be crushed under a load as of thirty of his fellows, and spread over the surface in a cake by a slow viscous yielding, like that which we see in glaciers, or tar, or other such semi-fluid mass! We have given this slight license to our fancy in order to test our readers. Those who have read it with proper disgust are invited to proceed with the article, where they will find no more of it; those who have been pleased with it are exhorted to turn from what must be henceforth to them a dull and dreary path, and betake themselves for their scientific instruction to the popular treatises of the day, where they will find it in copious streams, not generally diluted by more than a faint admixture of sense and of cold and stern science. The unit for measurement of work usually employed by engineers is the foot-pound; and, though this varies in amount from one locality to another, it is in such general use, and so convenient when absolute accuracy is not required, that we shall employ it throughout. It is the amount of work required to raise a pound a foot high. It is evident that to raise any mass to a given height, the amount of work required is proportional to the number of pounds in the mass, and also to the number of feet through which it is to be raised. Thus to raise a cwt. a furlong high requires the same expenditure of work (73,920 foot-pounds) as to raise a stone-weight a mile high, or a pound 14 miles. And the potential energy of the raised mass, or the work which can be got out of it in virtue of its position, is precisely equivalent to the work which has been employed in raising it. But if the mass be allowed to fall, we may remark that it gains velocity as it descends, and that the square of the velocity acquired at any point of the path is proportional to the space through which the mass has fallen. Also when a projectile is discharged vertically upwards, it possesses no potential energy at the commencement of its flight, but it has, in virtue of its motion, energy, or power of doing work. To measure this energy, we must find how much work it is capable of producing, and we find that it is proportional to the square of the velocity. That is, a projectile discharged upwards will rise to four times the height if its initial velocity be doubled, to nine times if trebled, and so on. Now if we introduce the term Kinetic Energy to signify the amount of work which a mass can do in virtue of its motion, we must measure it by half the product of the mass into the square of its velocity; and the ordinary formulæ for the motion of a projectile show that, neglecting the resistance of the air, the sum of the Potential and Kinetic Energies remains constant during the flight. There is per |