Watt, however, went to London, and, with the help of his own and of Boulton's influential friends, succeeded in getting his bill through. His patent was extended 24 years, and Boulton & Watt set about the work of introducing their engines with the industry and enterprise which characterized their every act.

In the new firm, Boulton took charge of the general business, and Watt superintended the design, construction, and erection of their engines. Boulton's business capacity, with Watt's wonderful mechanical ability-Boulton's physical health, and his vigor and courage, offsetting Watt's feeble health and depression of spirits-and, more than all, Boulton's pecuniary resources, both in his own purse and in those of his friends, enabled the firm to conquer all difficulties, whether in finance, in litigation, or in engineering.

It was only after the successful erection and operation of several engines that Boulton and Watt became legally partners. The understood terms were explicitly stated by Watt to include an assignment to Boulton of two-thirds the patent-right; Boulton paying all expenses, advancing stock in trade at an appraised valuation, on which it was to draw interest; Watt making all drawings and designs, and drawing one-third net profits.

As soon as Watt was relieved of the uncertainties regarding his business connections, he married a second wife, who, as Arago says, by "her various talent, soundness of judgment, and strength of character," made a worthy companion to the large-hearted and large-brained engineer. Thenceforward his cares were only such as every businessman expects to be compelled to sustain, and the next ten years were the most prolific in inventions of any period in Watt's life.

From 1775 to 1785 the partners acquired five patents, covering a large number of valuable improvements upon the steam-engine, and several independent inventions. The first of these patents covered the now familiar and univer

sally-used copying-press for letters, and a machine for drying cloth by passing it between copper rollers filled with steam of sufficiently high temperature to rapidly evaporate the moisture. This patent was issued February 14, 1780.

ว

In the following year, October 25, 1781, Watt patented five devices by which he obtained the rotary motion of the engine-shaft without the use of a crank. One of these was the arrangement shown in Fig. 27, and known as the "sun

and-planet" wheels. The crank-shaft carries a gear-wheel, which is engaged by another securely fixed upon the end of the connecting-rod. As the latter is compelled to revolve about the axis of the shaft by a tie which confines the connecting-rod end at a fixed distance from the shaft, the shaft-gear is compelled to revolve, and the shaft with it. Any desired velocity-ratio was secured by giving the two gears the necessary relative diameters. A fly-wheel was used to regulate the motion of the shaft.' Boulton & Watt used the sun-and-planet device on many engines, but finally adopted the crank, when the expiration of the patent held by Matthew Wasborough, and which had earlier date than Watt's patent of 1781, permitted them. Watt had proposed the use of a crank, it is said, as early as 1771, but Wasborough anticipated him in securing the patent. Watt had made a model of an engine with a crank and fly-wheel, and he has stated that one of his workmen, who had seen the model, described it to Wasborough, thus enabling the latter to deprive Watt of his own property. The proceeding excited great indignation on the part of Watt; but no legal action was taken by Boulton & Watt, as the overthrow of the patent was thought likely to do them injury by permitting its use by more active competitors and more ingenious men.

The next patent issued to Watt was an exceedingly important one, and of especial interest in a history of the development of the economical application of steam. This patent included:

1. The expansion of steam, and six methods of applying the principle and of equalizing the expansive power.

2. The double-acting steam-engine, in which the steam acts on each side the piston alternately, the opposite side being in communication with the condenser.

1 For the privilege of using the fly-wheel to regulate the motion of the engine, Boulton & Watt paid a royalty to Matthew Wasborough, who had patented it, and who held also the patent for its combination with a crank, as invented by Pickard and Steed.

3. The double or coupled steam-engine-two engines capable of working together, or independently, as may be desired.

4. The use of a rack on the piston-rod, working into a sector on the end of the beam, thus securing a perfect rectilinear motion of the rod.

5. A rotary engine, or "steam-wheel."

The efficiency to be secured by the expansion of steam had long been known to Watt, and he had conceived the idea of economizing some of that power, the waste of which was so plainly indicated by the violent rushing of the exhaust-steam into the condenser, as early as 1769. This was described in a letter to Dr. Small, of Birmingham, in May of that year. When experimenting at Kinneil, he had tried to determine the real value of the principle by trial on his small engine.

Boulton had also recognized the importance of this improved method of working steam, and their earlier Soho engines were, as Watt said, made with cylinders "double the size wanted, and cut off the steam at half-stroke." But, though "this was a great saving of steam, so long as the valves remained as at first," the builders were so constantly annoyed by alterations of the valves by proprietors and their engineers, that they finally gave up that method of working, hoping ultimately to be able to resume it when workmen of greater intelligence and reliability could be found. The patent was issued July 17, 1782.

Watt specified a cut-off at one-quarter stroke as usually best.

Watt's explanation of the method of economizing by expansive working, as given to Dr. Small,' is worthy of reproduction. He says: "I mentioned to you a method of still doubling the effect of steam, and that tolerably easy, by using the power of steam rushing into a vacuum, at

1 "Lives of Boulton and Watt," Smiles.

present lost. This would do a little more than double the effect, but it would too much enlarge the vessels to use it all. It is peculiarly applicable to wheel-engines, and may supply the want of a condenser where force of steam is only used; for, open one of the steam-valves and admit steam, until one-fourth of the distance between it and the next valve is filled with steam, shut the valve, and the steam will continue to expand and to pass round the wheel with a diminishing power, ending in one-fourth its first exertion. The sum of this series you will find greater than one-half, though only one-fourth steam was used. The power will indeed be unequal, but this can be remedied by a fly, or in several other ways."

It will be noticed that Watt suggests, above, the now well-known non-condensing engine. He had already, as has been seen, described it in his patent of 1769, as also the rotary engine.

Watt illustrates and explains his idea very neatly, by a sketch similar to that here given (Fig. 28).

Steam, entering the cylinder at a, is admitted until onefourth the stroke has been made, when the steam-valve is closed, and the remainder of the stroke is performed without further addition of steam. The variation of steampressure is approximately inversely proportional to the variation of its volume. Thus, at half-stroke, the pressure becomes one-half that at which the steam was supplied to the cylinder. At the end of the stroke it has fallen to onefourth the initial pressure. The pressure is always nearly equal to the product of the initial pressure and volume divided by the volume at the given instant. In symbols,

PV
P =
V

It is true that the condensation of steam doing work changes this law in a marked manner; but the condensation and reëvaporation of steam, due to the transfer of heat to