Steam Engine

Among other important devices associated with Watt was the "indicator," by which diagrams showing the relation of the steam pressure in the cylinder to the movement of the piston are auto matically drawn : its invention seems to have been mainly due to his assistant John Southern.

In partnership with Matthew Boulton, Watt carried on in Bir mingham the manufacture and sale of his engines with great suc cess, and held the field against all rivals in spite of severe assaults on the validity of his patents. Notwithstanding his knowledge of the advantage to be gained by using steam expansively, he con tinued to employ only low pressures—seldom more than 7 lb. per square inch over that of the atmosphere. His boilers were fed, as Newcomen's had been, through an open pipe which rose high enough to let the column of water in it balance the pressure of the steam. He gave a definite numerical significance to the term "horse-power," defining it as the rate at which work is done when 33,000 lb. are raised one foot in one minute.

In the fourth claim of Watt's first patent, quoted above, the second sentence describes a non-condensing engine, which would have required steam of a higher pressure. This, however, was a line of invention which Watt did not follow up, perhaps because so early as 1725 a non-condensing engine had been described by Jacob Leupold in his Theatrum machinarum.

It was not till much later that the thermodynamic principles underlying the action of the steam engine came to be understood. Engineers were consequently slow to appreciate the fact that to obtain economy of fuel it was advantageous to employ a high initial pressure, in combination with much expansion in the cylin der and with the separate condenser of Watt.

Trevithick, Bull and Evans.

The introduction of the non condensing and, at that time, relatively high-pressure engine was effected in England by Richard Trevithick and in America by Oliver Evans about i800. Both Evans and Trevithick applied their engines to propel carriages on roads, and both used for boiler a cylindrical vessel with a cylindrical flue inside containing the fire —the construction now known as the Cornish boiler. In associa tion with Edward Bull, Trevithick had previously made direct acting pumping-engines, with an inverted cylinder set over and in line with the pump-rod, thus dispensing with the beam that had been a feature in all earlier forms. But in these "Bull" engines, as they were called, the steam was condensed by a jet of cold water in the exhaust-pipe, and Boulton and Watt successfully opposed them as infringing Watt's patent. To Trevithick' belongs

the honour of being the first to use a steam carriage on a railway; in 1804 he built a locomotive in the modern sense, to run on what had formerly been a horse-tramway, in Wales. In this connection it may be added that as early as 1769 a steam carriage for roads had been built in France by Nicolas Joseph Cugnot, who used a pair of single-acting high-pressure cylinders to turn a driving axle step by step by means of pawls and ratchet-wheels. To the initia tive of Evans may be ascribed the early general use of high pressure steam in the United States, a feature which for many years distinguished American from English practice. (See Loco MOTIVE.) Compound Engine and Cornish Engine.—Among contem poraries of Watt the name of Jonathan Hornblower deserves special mention. In 1781 he constructed and patented what would now be called a compound engine, with two cylinders of different sizes. Steam was first admitted into the smaller cylinder, and then passed over into the larger, doing work against a piston in each. In Hornblower's engine the two cylinders were placed side by side, and both pistons worked on the same end of a beam overhead. This was an instance of the use of steam expansively, and as such was earlier than the patent, though not earlier than the invention, of expansive working by Watt. Hornblower was crushed by the Birmingham firm for infringing their patent in the use of a sepa rate condenser and air-pump. The compound engine was revived in 1804 by Arthur Woolf, with whose name it is often associated. Using steam of fairly high pressure, and cutting off the supply before the end of the stroke in the small cylinder, Woolf expanded the steam to several times its original volume. Mechanically the two-cylinder compound engine has some advantage over a one cylinder engine with the same amount of expansion, in exerting a more uniform driving effort. But another and more important merit of the system lies in the fact that by dividing the whole range of expansion into two parts the cylinders in which these are separately performed are subject to a reduced range of fluctuation in their temperature. This helps to limit a source of waste which is present in all piston engines, namely the waste which results from the heating and cooling of the metal by its alternate contact with hot and cooler steam. The introduction of compound ex pansion forms the most outstanding improvement which steam engines of the piston and cylinder type have undergone since the time of Watt.