that, although the boiler appeared to be of ample size, having regard to the dimensions of the cylinder, it was incompetent to generate sufficient steam to supply the heavy demand.
Watt was very much struck by this large consumption of steam, and at once turned his powerful mind to the consideration of how it was that so large a quantity of steam was needed. He saw it was due to the cold water used to condense the steam being injected into the very steam cylinder itself, and being played into that cylinder until its walls were brought down to a temperature corresponding to the vacuous condition intended to be produced in it; that, therefore, the quantity of incoming steam needed to fill the cylinder to atmospheric pressure in the up-stroke was not merely that represented by the cubic contents of the cylinder, but was, in addition, that needed in the first instance to heat up the whole of the walls of the cylinder, and the piston, with the water packing on the top of it, to its own temperature, to very considerably heat up the water accumulated in the cylinder, and also to expel the liquid contents and the air at the ‘snifting valve.’ Watt estimated these sources of loss as demanding at least three times as much steam as would have been needed to fill the contents of the cylinder; and, in actual practice, with large engines, in after years, he based his remuneration upon one-third of the cost of the fuel saved. At this time, and for some years previously, Joseph Black had held the chair of chemistry in Glasgow University, and in the course of his experiments had made the discovery of latent heat; that is to say, he had proved that mere temperature capable of being appreciated by a thermometer was by itself no guide as to the heat which had to be communicated to bodies to occasion changes of condition. This important scientific fact was repeatedly enunciated by Black in his lectures. Although it appears Watt had not the leisure to attend these lectures, he nevertheless was cognisant of the discovery, and he pursued the investigations into latent heat in connection with steam; he also determined the relation between the bulks of steam and water at atmospheric pressure, at pressures less than the atmosphere, and, to some extent, at pressures above the atmosphere. In fact, he prepared himself, as a man of science, to deal with the problem of improvements in the steam engine in actual practice. The solution of this problem by Watt was to condense the steam, not in the cylinder itself, but in a separate vessel, in connection, however, with the cylinder at appropriate times. The jet of cold water was thus from henceforth for ever discarded from entering the steam cylinder.
With the early models constructed by Watt the separate vessel was composed of thin metal and was immersed in water; in other words, it was the ‘surface condenser.’ But subsequently, although as a rule the condenser continued to be immersed in water, the main reliance was no longer placed upon the cooling of the sides, but upon the use in the separate condenser of such an injection as had been employed by Newcomen in the steam cylinder itself. It must strike every one (of course it at once occurred to Watt) that in a very short time his condenser would be full of water from the condensed steam, mixed with the incondensable air liberated from the steam and from the condensing water, and that thus the vacuous condition would be speedily lost. The remedy for this was to apply an ordinary pump, to pump out the condensed steam, and also, where injection was used, the water of condensation and the air, and in this way the separate vessel was at all times maintained in a partially vacuous condition. As has already been said, Watt's want of means, and the need of pursuing other avocations for a livelihood, retarded the practical outcome of the invention for some time. Indeed, the want of means even prevented the application for a patent to secure the invention; for, although the discovery was made in 1765, the patent was not obtained until 1769 (No. 913). It does not appear that in the preparation of the specification Watt had the benefit of legal advice, but he had plenty of friendly philosophical advice. As a result of this amateur assistance the specification was so clumsily drawn that the validity of the patent was, many years afterwards, seriously contested. This patent not only included the separate condenser, with the air-pump, but it also embraced a variety of other matters. In the specification there is enunciated the doctrine which is as truly at the root of all engine economy at the present day as it was in the days of Watt—namely, that the walls of the cylinder should be maintained at the same heat as that of the steam which is about to enter the cylinder. Watt proposed to do this by means of an external casing, leaving an annular space between it and the outside of the cylinder, in which space there should always be steam, this external casing to be itself surrounded by some non-conductor. It should have been stated that Watt experimented with wooden cylinders, hoping
