(80.)

Improved methods of boring soon, however, relieved [Pg149] the engine from a part of these imperfections, and Watt writes to Mr. Smeaton in the letter above quoted as follows:—

"Mr. Wilkinson has improved the art of boring cylinders; so that I promise, upon a 72 inch cylinder, being not further distant from absolute truth than the thickness of a thin sixpence in the worst part. I am labouring to improve the regulators; my scheme is to make them acute conical valves, shut by a weight, and opened by the force of the steam. They bid fair for success, and will be tried in a few days."

The person here alluded to was Mr. John Wilkinson, of Bersham near Chester, who, about the year 1775, contrived a new machine for accurately boring the insides of cylinders. The cylinder being first obtained from the foundery with a surface as accurate as the process of casting would admit, had its inner surface reduced to still greater accuracy by this machine, which consists of a straight central bar extended along the axis of the cylinder, which was made to revolve slowly round it. During the operation of boring, the borer or cutter was fitted to slide along this bar, which being perfectly straight, served as a sort of ruler to guide the borer or cutter in its progress through the cylinder. In this manner the interior surface of the cylinder was rendered not only true and straight in its longitudinal direction, but also perfectly circular in its cross section.

The grease found to be most eligible for lubrication was the tallow of beef or mutton; but in the earlier cylinders this was soon consumed by reason of the imperfection of the boring, and the piston being left dry ceased to be steam-tight. To prevent this, Watt sought for some substance, which while it would thicken the tallow, and detain it around the piston, would not be subject to decomposition by heat. Black lead dust was used for this purpose, but was soon found to wear the cylinder. In the mean while, however, the improved method of boring supplied cylinders which rendered this expedient unnecessary.

When the inner surface of the cylinder is perfectly true and smooth, the packing of the piston is soon rendered solid and hard, being moulded to the cylinder by working, so as to fit it perfectly. When by wear it became loose, it was [Pg150] only necessary to tighten the screws by which the top and bottom of the piston were held together. The packing being compressed by those means, was forced outwards towards the surface of the cylinder, so as to be rendered steam-tight.