This experiment succeeded perfectly, and confirm ed Mr. Watt's highest expectations. He therefore applied in 1768 for letters patent for " methods of lessening the consumption of steam, and consequently of fuel in steam engines, which passed the seals in Janu ary 1769. The specification recited seven different principles, viz.
I. To keep the steam cylinder as hot as the steam which enters it.
2. To condense the steam in separate vessels, to be kept as cold as the adjacent air by water or cold bodies.
3. To draw the air or vapour out of the cylinder, and condense by pumps wrought by the engine.
4. To employ the expansive force of steam to press upon the piston; and in cases where cold water is not plentiful, to work the engines by this force of steam only by discharging the steam into the open air after it has done its office.
5. Where rotatory motions are required, to make the steam vessels in the form of hollow rings, with inlets and outlets for the steam, mounted on horizon tal axles.
6. To apply a degree of cold to contract the steam, so that the engine may be wrought by the alternate contractions and expansion of the steam.
7. To render the parts of the engine, air, and steam tight, by using oils, wax, resinous bodies, fat of animals, quicksilver, and other metals in a fluid state.
While this patent was passing through its different stages, Mr. \Vatt was engaged in experiments with an engine which, with the assistance of Dr. Roebuck, who had purchased part of his patent right, he had erected at a coal mine near llorrowstounness. It had a cylinder 18 inches in diameter, and was successive ly altered and improved so as to embody several of the principles above mentioned. When these expe riments were finished, Mr. Wa.u. and Dr. Roebuck began their arrangements for manufacturing the en gines on a great scale, but pecuniary difficulties pre vented Dr. Roebuck from giving the most necessary aid, and he accordingly, in 1773, with Mr. Watt's consent, resigned his share in the patent to Mr. Mat thew Bolton, a man of generosity, enterprise, and talent, and every way fitted for bringing into action the talents and inventions of Mr. Watt.
The extent of the arrangements, and the length of time necessary to bring the improved steam engine before the public, were soon discovered by experience, and Mr. Watt saw that it was impossible to reimburse himself for the great outlay which these arrange ments rendered necessary during the few years of his patent which had yet to run.
He therefore applied to parliament in 1774 for an extension of the term of his patent; and with the zealous aid of Mr. Bolton, Dr. Roebuck, and Dr. Robison, he obtained in 1778 the exclusive privilege of manufacturing his improved engine for the space of twenty-five years.
About that time Mr. Watt and Mr. Bolton com menced a partnership for the manufacture of the im proved engine, which continued till the expiry of the exclusive privilege in 1800. At their establishment at Soho near Birmingham, many admirable engines were soon made; and erected in Staffordshire, Shrop shire,Warwickshire, Ste. and Messrs. Watt and Bolton
granted licences to use their engines, on the condition of securing a third part of the saving of coal, compared with an atmospheric engine performing the same work with coals equal in quantity. The amount of this saving was determined by ascertaining experimentally the coals consumed during any number of strokes made by the common and the improved engine. The num ber of strokes made in any given interval was ascer tained by a piece of machinery called the counter, which was struck at every ascent of the working beam. Two keys of this machine were kept, one by the patentees, and the other by the proprietors; and a traveller who examined it at stated times, calculated the saving of coal from the number of strokes.
These different inventions of Mr. Watt we shall now proceed to describe in their order.
Description of Mr. Trait's Single Reciprocating Engine, as constructed in 1788.
This engine is represented in Plate DVII. Fig. 1. The cylinder is shown at A surrounded with its steam case E. The cylinder, which is truly bored, is closed at its top by a cover, in the centre of which is the stuffing box D, through which the piston rod C moves. This stuffing hox is constantly supplied with melted tallow, and the piston rod being turned to a true cylinder, no steam can escape. The piston B is also made steam-tight, and for this purpose it is supplied with melted tallow, through a funnel in the top of the cylinder. The boiler for generating the steam is shown at the left hand side of the engine, where n is the boiler, p the grate, and o o the flues on the side of it, r the damper, s the chimney, t the feed-pipe by which the boiler is supplied with water, and I, 2, the guage-cocks. The steam pipe F proceeding from the boiler, and passing through the well, conveys the steam to the upper steam nozzle or valve G, through which it enters by the horizontal passage above the piston, and presses it, by its elastic force, down to the bottom of the cylinder. The perpendicular steam pipe which conveys the steam from the top of the cylinder into the bottom of it is shown at I. K is the equilibrium valve or nozzle, or the lower steam valve, which admits the steam below the piston, through the curved horizontal passage, to the left of L. When the valve is open there is an equilibrium between the steam entering both above and below the piston, from which circumstance it derives its name. I is the eduction pipe by which the steam is conveyed from the cylinder to the condenser M, which is a cylindri cal vessel surrounded by cold water. N is the injec tion cock, which admits a jet of cold water from the cistern to condense the steam in N. At 0 is seen the termination of a copper pipe, called the Snift or Blow valve It reaches to the outside of the cistern, and has its orifice closed with a little valve opening outwards and immersed in a small vessel of water. The object of it is to discharge the air from the vessels when the engine is first set a going.