On the progressive improvements made in the efficiency of steam engines in cornwall; with investigations of the methods best adapted for imparting great angular velocities

Abstract

The practical adaptation of the steam-engine to mechanical purposes is considered by the author as due to Mr. Newcomen, whose engines were introduced into Cornwall very early in the last century, and soon superseded the rude machinery which had till then been employed for raising water from the mines by the labour of men and of horses. The terms proposed by Mr. Watt, in virtue of his patent in 1769, which secured to him, until the year 1800, the receipts of one third of all the savings in fuel resulting from the adoption of his improvements in the construction of the engine, rendered it necessary to institute an accurate comparison between the efficiency of his with former engines. A copy of the report drawn up on this occasion, in October 1778, is given in the paper; but as the dynamic unit of one pound avoirdupois, raised through a height of one foot, had not yet been established as the measure of efficiency, the author, proceeding upon the data furnished by that report, calculates that the duty performed by Watt’s engine, with the consumption of one bushel of coal, on that occasion was 7,037,800. In the year 1793, an account was taken of the work performed by seventeen engines on Mr. Watt’s construction, then working in Cornwall, their average duty was 19,569,000; which exceeds the performance of the former atmospheric engines, in the standard experiments, in the proportion of 2·78 to 1. Some years afterwards, disputes having arisen as to the real performance of Mr. Watt’s engines, the matter was referred to five arbiters, of whom the author was one; and their report, dated in May 1798, is given as far as relates to the duties of the engines. The general average of twenty-three engines was 17,671,000. Since that period, so great have been the improvements in the economy of fuel and other parts of the machinery, that in December 1829, the duty of the best engine, with a cylinder of 80 inches, was 75,628,000, exceeding the duty performed in 1795, in the proportion of 3·865 to 1; and that of the atmospheric engine of 1778, in the proportion of 10·75 to 1. The remainder of the paper relates to the friction in machinery, and the different modes of obviating its effects. With a view of reducing the amount of friction, the author is led to consider what are the most proper forms for the teeth and cogs of wheels; and through what intermediate steps a given increase of angular velocity may be most advantageously communicated. Equability of velocity is obtained, though at the expense of some degree of sliding friction, when the outline of the teeth of the wheels are involutes of circles. Friction, on the other hand, is wholly prevented when their form is the logarithmic spiral; but the angular velocities will then be variable. Hence these two advantages are incompatible with one another; but on the whole, the author gives the preference to the involute, which produces an equability of angular motion. The most advantageous mode of increasing velocity by a series of wheels is to adjust them so that the multiplication of velocity shall proceed in a geometrical progression.