Mr. Watt very early found that, although most kinds of grease would answer when employed to keep the piston tight, yet that beef or mutton tallow were the most proper, and the least liable to decompose ; but when cylinders were new and imperfectly bored, the grease soon disappeared, and the piston was left dry ; he therefore endeavored to detain it by thicken ing it with some substance which would lubricate the cylinder, and not prove decomposable by heat and exhaustion. Black-lead dust seemed a proper sub stance, and was therefore employed, especially when a cylinder or the packing of a piston was new ; but it was found in the sequel that the black-lead wore the cylinder, though slowly ; and by more perfect work manship, cylinders are made so true as not to require it, or at least, only for a very short time at first using.
The joints of the cylinder, and other parts of New. comen's engines, were generally made tight by being screwed together upon rings of lead covered with gla zier's putty, which method was sufficient, as the en try of small quantities of air did not materially af fect the working of these engines where only a very imperfect exhaustion was required. But the contrary being the case in the improved engines, this method would not answer Mr. Watt's purpose. He at first made his joints very true, and screwed them together upon pasteboard, softened by soaking them in water, which answered tolerably well for a time, but was not sufficiently durable. He therefore endeavoured to find out some more lasting substance ; and observing that at the iron founderies they filled up flaN,s by iron bor ings or filings, moistened by urine, which in time became hard, he improved upon this by mixing the iron borings or filings with a small quantity of sul phur, and a little sal-ammoniac, to which he after wards added some fine sand from the grindstone troughs. This mixture, being moistened with water and spread upon the joint, heats soon after it is screwed together, becomes hard, and remains good and tight for years, which has contributed in no small degree to the perfection of the engines.
Mr. Murdock, much about the same time, without communication. with Mr. Watt, made a cement of iron borings and sal-ammoniac, without the sulphur, But the latter gives the valuable property of making the cement set immediately.
The following is an account of the actual perform ance of some of Mr. Watt's engines, as given by him self : The burning of one bushel of good Newcastle or Swansea coals in Mr. Watt's reciprocating engines,
working more or less expansively, was found, by the accounts kept at the Cornish mines, to raise from 24 to 32 millions of pounds of water one foot. high : the greater or less effect depending upon the state of the engine, its size, and rate of working, and upon the quality of the coal.
In engines upon the rotative double construction, one having a cylinder of 31 inches diameter, and making 171 strokes of 7 feet long per minute, called 40 horses' power,* meaning the constant exertion of 40 horses (for which purpose, supposing the work to go on night and day, 3 relays, or at least 120 horses, must be kept) consumed about 4 bushels of good New castle coal per hour, or 400 weight of good Wednes bury coal. A rotative double engine, with a cylinder of 23i inches in diameter, making 211 strokes of 5 feet long per minute, was called '20 horses' power ; and an engine, with a cylinder of 171 inches diame ter, making 25 strokes of 4 feet long per minute, was called 10 horses' power ; and the consumption of coals by these was nearly proportional to that of the 40 horses' power.
A bushel of Newcastle coals, which thus appears to he the consumption of a 10 horse engine for one hour, grinds and dresses about 10 bushels, Winches ter measure, of wheat.
The quantity of water necessary for injection may be determined on principle for engines having a sepa rate condenser. Having found the contents of the cylinder in cubic feet (that is, the area of the piston multiplied by the length of the stroke + to allow for the vacuities at top and bottom through which the piston does not pass,) it is to be considered that every cubic foot of steam produces about a cubic inch of water when condensed, and contains about as much latent heat as would raise 960 cubic inches of water one degree. This steam must not only be condensed, but must be cooled down to the temperature of the hot well : therefore as many inches of cold water must be employed as will require all this heat to raise it to the temperature of the hot well.
Therefore let c be the quantity of steam to be con densed in cubic feet; a =the temperature of the cold water (per Fahrenheit); b = the proposed temperature of the warm water, or hot well; 1172 = the sensible and latent heat of steam; x = the cubic inches of cold water required to con dense c.