By Denny Lane, of Cork.

Among the most useful inventions of the latter half of the nineteenth century the gas-engine holds a prominent place. While its development has not been so brilliant or so startling as that which we can note in the employment of electricity, it holds, among the applications of heat, the most important place of any invention made within that period. Even amid the contrivances by which, in recent times, the other forces of nature have been subdued to the uses of man, there are only a few which rival the gas-engine in practical importance. With regard to the steam-engine itself, it is remarkable how little that is new has really been invented since the time of Watt and Woulfe. In the specifications of the former can be shown completely delineated, or fully foreshadowed, nearly every essential condition of the economy and efficiency attained in our own days; and it is only by a gradual "survival of the fittest" of the many contrivances which were made to carry out his broad ideas that the steam-engine of the present has attained its great economy.

It is but within the last fifty years that the laws of the relation between the different physical forces were first enunciated by Justice Grove, and confirmed by the classical researches of Dr. Joule - the one a lawyer, working hard at his profession, the other a man of business engaged in manufacture. Both are still living among us; the latter having withdrawn from business, while the former is a Judge of the High Court of Justice. I always regret that the claims of his profession have weaned Justice Grove from science; for, while it may be possible to find in the ranks of the Bar many who might worthily occupy his place on the Bench, it would be hard to find among men of science any with as wide-reaching and practical philosophy as that which he owns. The chemist demonstrated long since that it was impossible for man to create or destroy a single particle of ponderable matter; but it remained for our own time to prove that it was equally impossible to create or destroy any of the energy which existed in nature as heat, mechanical power, electricity, or chemical affinity. All that it is in the power of man to do is to convert one of these forms into another.

This, perhaps the greatest of all scientific discoveries since the time of Newton, was first, I believe, enunciated in 1842 by Grove, in a lecture given at the London Institution; and it was experimentally proved by the researches of Joule, described in a paper which he read at the meeting of the British Association which was held at Cork - my native city - in 1843. My friend Dr. Sullivan, now President of Queen's College, Cork, and I myself had the privilege of being two of a select audience of half a dozen people, who alone took sufficient interest in the subject to hear for the first time developed the experimental proof of the theory which welds into one coherent system the whole physical forces of the universe, and enables one of these to be measured by another. One branch of the "correlation of physical forces," as it was termed by Grove, was the relation between mechanical power and heat, and the convertibility of each into the other, which, under the name of "Thermodynamics," has become one of the most important branches of practical science.

Joule's first experiments clearly proved that each of these forms of energy was convertible into the other; but some discrepancies arose in determining the exact equivalent of each. His subsequent researches, however, clearly demonstrated the true relation between both. Taking as the unit of heat the amount which would be necessary to raise 1 lb. of water 1° of Fahrenheit's scale (now called "the English thermal unit"), he proved that this unit was equivalent to the mechanical power which would be required to raise 772 lb. 1 foot, or to raise 1 lb. 772 ft. perpendicularly against the force of gravity. The heat-unit - the pound-degree - which I will distinguish by the Greek letter θ, is a compound unit of mass and temperature; the second - the foot-pound = f.p. - a compound unit of mass and space. This equation, called "Joule's equivalent," or 1 thermal unit = 772 foot-pounds, is the foundation and the corner-stone of thermodynamics.

It is essential to understand the meaning of this equation. It expresses the maximum effect of the given cause, viz., that if all the heat were converted into power, or all the power were converted into heat, 1 thermal unit would produce 772 foot-pounds, or 772 foot-pounds would raise 1 lb. of water 1° Fahr. But there is never a complete conversion of any form of energy. Common solid coal may be partly converted into gases in a retort; but some of the carbon remains unchanged, and more is dissipated but not lost. In the same way, if I take five sovereigns to Paris and convert them into francs, and return to London and convert the francs into shillings, I shall not have 100 shillings, but only perhaps 95 shillings. But the five shillings have not been lost; three of them remain in the French change de monnaies, and two of them in the English exchange office. I may have forfeited something, but the world has forfeited nothing. There remains in it exactly the same number of sovereigns, francs, and shillings as there was before I set out on my travels.

Nothing has been lost, but some of my money has been "dissipated;" and the analogous case, "the dissipation of energy," has formed the subject of more than one learned essay.

Before the invention of the steam-engine, the only powers employed in mechanics were those of wind and water mills, and animal power. In the first two, no conversion of one force into another took place; they were mere kinematic devices for employing the mechanical force already existing in the gale of wind and the head of water. With regard to the power developed by man and other animals, we had in them examples of most efficient heat-engines, converting into power a large percentage of the fuel burnt in the lungs. But animal power is small in amount, and it is expensive for two reasons - first, because the agents require long intervals of rest, during which they still burn fuel; and next, because the fuel they require is very expensive. A pound of bread or beef, or oats or beans, costs a great deal more than a pound of coal; while it does not, by its combustion, generate nearly so much heat. The steam-engine, therefore, took the place of animal power, and for a long time stood alone; and nearly all the motive power derived from heat is still produced by the mechanism which Watt brought to such great efficiency in so short a time.