Murdock was also the inventor of the well-known cast-iron cement, so extensively used in engine and machine work. The manner in which he was led to this invention affords a striking illustration of his quickness of observation. Finding that some iron-borings and sal-ammoniac had got accidently mixed together in his tool-chest, and rusted his saw-blade nearly through, he took note of the circumstance, mixed the articles in various proportions, and at length arrived at the famous cement, which eventually became an article of extensive manufacture at the Soho Works. Murdock's ingenuity was constantly at work, even upon matters which lay entirely outside his special vocation. The late Sir William Fairbairn informed us that he contrived a variety of curious machines for consolidating peat moss, finely ground and pulverised, under immense pressure, and which, when consolidated, could be moulded into beautiful medals, armlets, and necklaces. The material took the most brilliant polish and had the appearance of the finest jet. Observing that fish-skins might be used as an economical substitute for isinglass, he went up to London on one occasion in order to explain to brewers the best method of preparing and using them. He occupied handsome apartments, and, little regarding the splendour of the drawing-room, he hung the fish-skins up against the walls. His landlady caught him one day when he was about to bang up a wet cod's skin! He was turned out at once, with all his fish. While in town on this errand, it occurred to him that a great deal of power was wasted in treading the streets of London! He conceived the idea of using the streets and roadways as a grand tread-mill, under which the waste power might be stored up by mechanical methods and turned to account. He had also an idea of storing up the power of the tides, and of running water, in the same way. The late Charles Babbage, F.R.S., entertained a similar idea about using springs of Ischia or of the geysers of Iceland as a power necessary for condensing gases, or perhaps for the storage of electricity.[12] The latter, when perfected, will probably be the greatest invention of the next half century. Another of Murdock's' ingenious schemes, was his proposed method of transmitting letters and packages through a tube exhausted by an air-pump. This project led to the Atmospheric Railway, the success of which, so far as it went, was due to the practical ability of Murdock's pupil, Samuel Clegg. Although the atmospheric railway was eventually abandoned, it is remarkable that the original idea was afterwards revived and practised with success by the London Pneumatic Dispatch Company.