When Mr. Watt's attention was turned to the sub ject of steam in 1759, it was then an effective and'use ful machine, and was used to a considerable extent in the mines and manufactories of the kingdom; but though it was then an effective machine, it was a very imperfect one, and required for its improvement all the energies of a mind deeply imbued with mechanical and chemical knowledge. In the year 1761 or 1762, Mr. Watt had constructed a model, with which he showed the practicability of what is now called the high pressure engine; but it was not till 1763, when he was repairing a model of Newcomen's engine, be longing to the college of Glasgow, that his mind was usefully directed to the subject. Having repaired it merely as a mechanician, he found, upon setting it to work, that its boiler, though apparently quite large enough,* could not supply it with steam. This he found to be caused by the small cylinder exposing a greater surface to condense the steam, than the cylinders of larger engines did in proportion to their respective contents. The cylinder of the model also, which was of brass, conducted heat better than the cast iron cylinders of larger engines, (generally cover ed on the inside with a stony crust) and hence Mr.
Watt conceived the idea of making this cylinder of wood baked to dryness, and soaked in linseed oil. He soon found, however, that the steam which was con densed in filling it exceeded the proportion of that re quired for large engines, according to Desaguliers. This effect Mr. Watt ascribed to the fact (newly dis covered by Dr. Cullen and some other philosophers) that water boiled in vacuo at heats below 100°, be cause at greater heats the water in the cylinder would generate steam, which would contribute to resist the pressure of the atmosphere.
In the progress of his experiments, Mr. Watt ascer tained that one cubic inch of water formed about a cubic foot, or 1728 cubic inches of ordinary steam, and that the condensation of that quantity of steam would raise six cubic inches. of water from the tem perature of the atmosphere to the boiling point. Hence he concluded that six times this rise Of temperature, or about 800° of heat, had been actually employed in the conversion of the water into steam, and all of which must be withdrawn before a perfect vacuum could be formed under the piston. Struck with this remarkable fact, and not understanding the reason of it, he mentioned it to his friend Dr. Black, who then explained to him his doctrine of latent heat, which he had for some time before this (summer of 1764) taught in the university.
Mr. Watt now perceived that, in order to make the best use of steam, two things were necessary. First, to maintain the cylinder as hot as the steam which entered it; and secondly, to cool down to 100°, and lower, if possible, the water produced from the con densation of the steam, and the injection water itself.
The means of accomplishing these two leading objects did not immediately present themselves, but early in 1765 it occurred to him that if a communication were opened between a cylinder containing steam, and another vessel exhausted of air and other fluids, the steam would immediately rush into the empty vessel, and continue to do so till it had established an equilibrium, and if that vessel were kept very cool by an injection or otherwise, more steam would continue to enter until the whole was condensed. Thus did Mr. Watt discover the great principle of condensation in a separate vessel;t but in its application he had to contend with difficulties which required new resources. As both the vessels were exhausted, or nearly so, how was the injection water, the air which would enter it, and the con densed steam, to be got out. This he proposed to ef fect in two ways: one of these ways was to adapt to the second vessel a pipe reaching downwards more than 34 feet, by which the water would descend, (a column of that length overbalancing the atmosphere) and by extracting the air with a pump. The second way was to employ one or more pumps to extract both the air and the water, which would be applica ble in all places, and essential in those cases where there was no well or pit. Mr. Watt preferred and invariably used the latter method.
In order to lubricate and keep the piston steam tight, Mr. Watt employed wax tallow or any other grease. This was effected by NCWCOMC11, by having water above the but when any of it entered the partially exhausted and hot cylinder it boiled, and prevented the production of a vacuum, besides cool ing the cylinder by its evaporation during the descent of the piston. As the mouth of the cylinder was open, Mr. Watt found that.the air which entered to act on the piston cooled the cylinder, and condensed some steam in again filling it. Ile, therefore, proposed to put an air-tight cover on the cylinder, through which cover there was a hole with a stuffing box for the piston rod to slide through; and when this suggested itself, it immediately occurred to him to admit the steam above the piston, to act upon it instead of the atmosphere. Another source of the loss of steam still remained, namely, the cooling of the cylinder by the external air, which would occasion at every stroke an internal condensation whenever the steam entered it. Mr. Watt proposed to provide against this, by placing the working cylinder within another cylinder containing steam, and to surround this by another of wood or of some bad conductor of heat. " When once," says Mr. Watt, " the idea of the separate con densation was started, all these improvements follow ed as corollaries in quick succession, so that in the course of one or two days the invention was thus far complete in my mind; and I immediately set about an experiment to verify it practically.