Passing from the consideration of water as a motive power in its natural state, I ask you to notice briefly the gigantic force it can be made to develop under the action of heat. In its normal form the power of water is due, as I have said, to its incompressibility; in the state of vapour, to which it is reduced by heat, its power is due to the counter force of expansion. It was when confined as a state prisoner in the Tower of London that the Marquis of Worcester began to speculate on the possibilities of steam, though he little dreamed of its more important applications, and the incalculable services it might be made to render to the cause of humanity. Suddenly, one day, his musings in his solitude were interrupted by the rattling of the lid of a kettle, which was boiling away on the fire beside him, when, being of a philosophic vein, he commenced to inquire after the cause; and he soon reasoned himself into the conclusion that the motive power lay in the tension of the vapour, and that the maintenance of this must be due to successive additions of heat. The thought was a seed sown in a fit soil, for it led to experiments which confirmed the supposition, and inaugurated others that have borne fruit, as we see. It was a great moment in the annals of discovery, and from that time to this the genius of improvement has moved onward with unprecedented strides; and this in the application of steam-power as well as the results, stupendous as these last have been. For as there is no department of industry that has not made immense advances since, none on which steam has not directly or indirectly been brought to bear with effect; so there has been no end to the ingenuity and ingenious devices by which steam has been coaxed into subjection to human use and made the pliant minister of the master, man. All these results follow as a natural consequence from the first discovery of its motive power by the Marquis of Worcester, and the subsequent invention of James Watt, by which the force detected was rendered uniform, instead of fitful and spasmodic, as it had been before. And yet, important as was the discovery of the one, and ingenious as is the invention of the other, both are of slight account in the presence of the great fact of nature observed by the English nobleman and humoured by the Scottish artisan. The _genie_ whom the one captured and the other tamed, is the great magic worker, apart from whose subtle strength their ingenuity had been wasted, and had come to naught.