When thousands of engines already were transforming heat into motion under the eyes of hundreds of professors, and when they had done so for half a century, or more; when thousands of trains, stopped by powerful brakes, were disengaging heat and spreading sheaves of sparks on the rails at their approach to the stations; when all over the civilised world heavy hammers and perforators were rendering burning hot the masses of iron they were hammering and perforating--then, and then only, Séguin, senior, in France, and a doctor, Mayer, in Germany, ventured to bring out the mechanical theory of heat with all its consequences: and yet the men of science ignored the work of Séguin and almost drove Mayer to madness by obstinately clinging to their mysterious caloric fluid. Worse than that, they described Joule’s first determination of the mechanical equivalent of heat as “unscientific.” When thousands of engines had been illustrating for some time the impossibility of utilising all the heat disengaged by a given amount of burnt fuel, then came the second law of Clausius. When all over the world industry already was transforming motion into heat, sound, light, and electricity, and each one into each other, then only came Grove’s theory of the “correlation of physical forces”; and Grove’s work had the same fate before the Royal Society as Joule’s. The publication of his memoir was refused till the year 1856. It was not the theory of electricity which gave us the telegraph. When the telegraph was invented, all we knew about electricity was but a few facts more or less badly arranged in our books; the theory of electricity is not ready yet; it still waits for its Newton, notwithstanding the brilliant attempts of late years. Even the empirical knowledge of the laws of electrical currents was in its infancy when a few bold men laid a cable at the bottom of the Atlantic Ocean, despite the warnings of the authorised men of science.