“The steam-pipe,” says Watt, “was adjusted to a small boiler. When steam was produced, it was admitted into the cylinder, and soon issued through the perforation of the rod, and at the valve of the condenser; when it was judged that the air was expelled, the steam-cock was shut, and the air-pump piston-rod was drawn up, which leaving the small pipes of the condenser in a state of vacuum, the steam entered them and was condensed. The piston of the cylinder immediately rose and lifted a weight of about 18 lbs., which was hung to the lower end of the piston-rod. The exhaustion-cock was shut, the steam was readmitted into the cylinder, and the operation was repeated. The quantity of steam consumed and the weights it could raise were observed, and, excepting the non-application of the steam-case and external covering, the invention was complete, in so for as regarded the savings of steam and fuel.”
WATT’S APPARATUS.
But, although the invention was complete in Watt’s mind, it took him many long and laborious years to work out the details of the engine. His friend Robison, with whom his intimacy was maintained during these interesting experiments, has given a graphic account of the difficulties which he successively encountered and overcame. He relates that on his return from the country, after the College vacation in 1765, he went to have a chat with Watt and communicate to him some observations he had made on Desaguliers’ and Belidor’s account of the steam-engine. He went straight into the parlour, without ceremony, and found Watt sitting before the fire looking at a little tin cistern which he had on his knee. Robison immediately started the conversation about steam, his mind, like Watt’s, being occupied with the means of avoiding the excessive waste of heat in the Newcomen engine. Watt, all the while, kept looking into the fire, and after a time laid down the cistern at the foot of his chair, saying nothing. It seems that Watt felt rather nettled at Robison having communicated to a mechanic of the town a contrivance which he had hit upon for turning the cocks of his engine. When Robison therefore pressed his inquiry, Watt at length looked at him and said briskly, “You need not fash yourself any more about that, man; I have now made an engine that shall not waste a particle of steam. It shall all be boiling hot,—ay, and hot water injected, if I please.” He then pushed the little tin cistern with his foot under the table.
Robison could learn no more of the new contrivance from Watt at that time; but on the same evening he accidentally met a mutual acquaintance, who, supposing he knew as usual the progress of Watt’s experiments, observed to him, “Well, have you seen Jamie Watt?” “Yes.” “He’ll be in fine spirits now with his engine?” “Yes,” said Robison, “very fine spirits.” “Gad!” said the other, “the separate condenser’s the thing: keep it but cold enough, and you may have a perfect vacuum, whatever be the heat of the cylinder.” This was Watt’s secret, and the nature of the contrivance was clear to Robison at once.
DR. JOSEPH BLACK.
It will be observed that Watt had not made a secret of it to his other friends. Indeed Robison himself admitted that one of Watt’s greatest delights was to communicate the results of his experiments to others, and set them upon the same road to knowledge with himself; and that no one could display less of the small jealousy of the tradesman than he did. To his intimate friend, Dr. Black, he communicated the progress made by him at every stage; and the Doctor kindly encouraged him in his struggles, cheered him in his encounter with difficulty, and, what was of still more practical value at the time, he helped him with money to enable him to prosecute his invention. Communicative though Watt was disposed to be, he learnt reticence when he found himself exposed to the depredations of the smaller fry of inventors. Robison says that had he lived in Birmingham or London at the time, the probability is that some one or other of the numerous harpies who live by sucking other people’s brains, would have secured patents for his more important inventions, and thereby deprived him of the benefits of his skill, science, and labour. As yet, however, there were but few mechanics in Glasgow capable of understanding or appreciating the steam-engine; and the intimate friends to whom he freely spoke of his discovery were too honourable-minded to take advantage of his confidence. Shortly after, Watt fully communicated to Robison the different stages of his invention, and the results at which he had arrived, much to the delight of his friend.
It will be remembered that in the Newcomen engine the steam was only employed for the purpose of producing a vacuum, and that its working power was in the down stroke, which was effected by the pressure of the air upon the piston; hence it is now usual to call it the atmospheric engine. Watt perceived that the air which followed the piston down the cylinder would cool the latter, and that steam would be wasted in re-heating it. In order, therefore, to avoid this loss of heat, he resolved to put an air-tight cover upon the cylinder, with a hole and stuffing-box for the piston-rod to slide through, and to admit steam above the piston, to act upon it instead of the atmosphere. When the steam had done its duty in driving down the piston, a communication was opened between the upper and lower part of the cylinder, and the same steam, distributing itself equally in both compartments, sufficed to restore equilibrium. The piston was now drawn up by the weight of the pump-gear; the steam beneath it was then condensed in the separate vessel so as to produce a vacuum, and a fresh jet of steam from the boiler was let in above the piston, which forced it again to the bottom of the cylinder. From an atmospheric it had thus become a true steam-engine, and with a much greater economy of steam than when the air did half the duty. But it was not only important to keep the air from flowing down the inside of the cylinder: the air which circulated within cooled the metal and condensed a portion of the steam within; and this Watt proposed to remedy by a second cylinder, surrounding the first with an interval between the two which was to be kept full of steam.
One by one these various contrivances were struck out, modified, settled, and reduced to definite plans; the separate condenser, the air and water pumps, the use of fat and oil (instead of water as in the Newcomen engine) to keep the piston working in the cylinder air-tight, and the enclosing of the cylinder itself within another to prevent the loss of heat. They were all but emanations from the first idea of inventing an engine working by a piston, in which the cylinder should be kept continually hot and perfectly dry. “When once,” says Watt, “the idea of separate condensation was started, all these improvements followed as corollaries in quick succession; so that in the course of one or two days the invention was thus far complete in my mind.”