The Stoker's Catechism

by W. J. Connor

41. Question.—Give a description of the Sun and Planet method, and why he invented it?

Answer.—The sun and planet were two cog-wheels geared into each other, the sun being 3 feet diameter and the planet 2 feet diameter, the latter was keyed tightly on the bottom end of the connecting rod, and the sun which was keyed tightly on the end of the shaft, that was to revolve and work the machinery. But although this method did make the machinery revolve, it was not smoothly, for when the planet wheel was at either top or bottom of the sun wheel, the power of the engine was less effective than it was half way in the opposite positions. This led Watt to add a large wheel on the shaft of the sun wheel, called the fly wheel, which equalised the rate of motion to uniformity. Watt invented the crank for his engine, but one of his men gave the tip to an engineer at Bristol, who forthwith took out a patent for it and forestalled Watt, who had to invent another means—the sun and planet. But when the term of the patent expired, Watt resumed the crank method instead of the sun and planet, which was noisy, the wear and tear very great, and also expensive.

42. Question.—What other things did Watt do towards the perfection of the steam engine?

Answer.—He added the air pump to his engine to draw the condensed steam and water from his separate condenser; he invented the throttle valve and the governor, in order to sustain a uniform rate of speed in the engine, whatever pressure of steam might be on, or variation of work, whether heavy or light.

43. Question.—Why is the power of the engine called horse-power?

Answer.—Before the invention of the engine, horses were employed in mills and mines, and other places; the number of horses employed in a mill or mine, indicating the amount of work going on, and the necessity of employing them, and when the steam engine came on the scene, and a purchaser wanted, he was told that the engine was equal to so many horses; that comparison gave the purchaser a clear idea of the engine he required. Savory was the first to suggest this comparison, but Watt knew that horses differed in size and strength, and in order to be sure of a safe standard for his engine power he experimented with big horses in some London breweries, and after careful calculation and comparison he fixed a horse-power at 32,000 lb., that is to say, that a horse could lift that weight of water one foot above the ground in a minute for eight hours per day. This standard has remained ever since, although it is above the average of the power of the average horse, it is in favour of the purchaser of an engine, as well as being capable of working more than eight hours a day, or twenty hours if required.

44. Question.—What is meant by "nominal horse-power"?

Answer.—It is a rough and ready way of giving some idea of the power of an engine or engines on the basis of the number of inches in the area of the cylinder or cylinders, but when the process of taking the diagram of the engine is gone through the term nominal is dropped, and indicated horse-power is then expressed, because it was proved by actual experiment and certainty.

45. Question.—How is that performance accomplished?

Answer.—In horizontal engines there are generally two gun-metal screw-plugs on the top of the cylinder, one over each end and in front of the piston; when a diagram is to be taken, these plugs are taken out and other screws put in their places, to which a copper pipe is attached; the screw plugs are 1 inch in diameter, also the copper pipes; and exactly mid-way on the copper pipe is a small cylinder which moves on a pivot, by means of a string with a turn round it. One end of the string is fixed by a clip on the connecting rod, the other end anywhere to keep the string tight, so that by the movement of the steam entering the cylinder at either end, and the connecting rod working backwards and forwards, the small cylinder is made to turn frontways and backways; and within the small cylinder is another cylinder very much smaller; it has a tiny piston within it, and as the steam presses on the little piston at every stroke of the engine, a pencil from the outer cylinder is fixed in a slot and marks the movements of the little piston on a roll of prepared paper, slid over the inner cylinder for that purpose, the pencil being kept up to the paper by means of a small steel spring. This diagram on the paper cylinder, not only is used for determining the power of the engine, but for detecting any irregularity in the slide-valve movements. Every hour during the trial the finished diagram is torn off the roll and a fresh one started, and when time is up the engine is stopped and the diagrams compared. Then commence the calculations, which are gone through somewhat in this manner: the common multiplier is found by multiplying the area of the piston in inches by the speed of the piston in feet per minute and the product divided by 32,000 (Watt's horse-power), then the effective mean pressure found on the diagram is multiplied by the common multiplier, and the quotient will be the indicated horse-power of the engine.