Watt at first intended adopting the expedient which had worked satisfactorily with the less effective condensation of Newcomen's engine--i. e., leading a pipe from the condenser to a depth greater than the height of a column of water which could be counterbalanced by the pressure of the atmosphere; but he subsequently employed the air-pump, which relieves the condenser not only of the water, but of the air which also usually collects in considerable volume in the condenser, and vitiates the vacuum. He next substituted oil and tallow for water in the lubrication of the piston and keeping it steam-tight, in order to avoid the cooling of the cylinder incident to the use of the latter. Another cause of refrigeration of the cylinder, and consequent waste of power in its operation, was seen to be the entrance of the atmosphere, which followed the piston down the cylinder at each stroke, cooling its interior by its contact. This the inventor concluded to prevent by covering the top of the cylinder, allowing the piston-rod to play through a "stuffing-box"--which device had long been known to mechanics. He accordingly not only covered the top, but surrounded the whole cylinder with an external casing, or "steam-jacket," and allowed the steam from the boiler to pass around the steam-cylinder and to press upon the upper surface of the piston, where its pressure was variable at pleasure, and therefore more manageable than that of the atmosphere. It also, besides keeping the cylinder hot, could do comparatively little harm should it leak by the piston, as it could be condensed, and thus readily disposed of. When he had concluded to build the larger experimental engine, Watt determined to give his whole time and attention to the work, and hired a room in an old deserted pottery near the Broomielaw. Here he worked with a mechanic--John Gardiner, whom he had taken into his employ--uninterruptedly for many weeks. Meantime, through his friend Dr. Black, probably, he had made the acquaintance of Dr. Roebuck, a wealthy physician, who had, with other Scotch capitalists, just founded the celebrated Carron Iron-Works, and had opened a correspondence with him, in which he kept that gentleman informed of the progress of his work on the new engine.