The interval which had thus elapsed between the date of the patent and the formation of the new partnership, occasioned Watt much alarm, lest the duration of the term should expire before the patent could be made profitable, or reimburse him the expenses incident to the arrangements which were necessary to the manufacture of the engines. At the suggestion of Bolton and other friends, and supported by their influence, he applied to Parliament for an extension of the term, which, after a slight opposition, was granted for 24 years, to be computed from the time of that application, namely, 1775 ; an extension highly creditable to the legislature which could foresee the commercial value of the invention as thus improved, and reward the man to whose genius it'was made valuable by granting to him so prolonged a monopoly. And it may be here remarked — such was the prejudice, the doubt, or the dislike to novelty, on the part of the public—the sum of very nearly 50,0001. was expended by Bolton and Watt in the manufacture of the improved engines before they realized any return !. This fact is a very sufficient extenuation for the somewhat dangerous precedent which the parliament afforded by its liberal extension of individual privilege to an inventor.
To give a detailed account or illustrative diagrams of the various machines erected from time to time shout this period by Mr. Watt would, in this rapid sketch of the early history of the steam-engine, and our necessarily restricted space, be quite impossible ; new modifications and diversities of arrangement were contrived by him to meet local exigencies according to the various situ ations where his engines were required, and to the work they had to perform. Of course improvement grew upon improvement as they became multiplied in use, and as no one engine was the exact counterpart of any that had been pre viously erected, it wilt be sufficient to explain the nature and object of his principal contrivances by the simplest illustrations.
Let A represent the cylinder (in which the piston h works) externally surrounded by a second cylinder or jacket b b, leaving a small space g, all round betwben the two; this apace g communicates by a large pipe f, with the boiler, and always full of steam, so as to keep the cylinder A at the same heat with the steam to be admitted, and to prevent its condensation ; the jacket b is furnished with a lid c, which has a hole in the centre for the piston rod a (con nected with the beam) to pass through. This rod is made truly cylindrical, so that the hole can be kept steam-tight by a collar of oakum screwed round it at e. The inner cylinder A has a close bottom, and the jacket 15 joins to the same; but the top of the internal cylinder does not reach quite up to the lid of the outer cylinder or jacket b, by which means the steam has always free access to the top of the piston h, from the space g between the cylinders, and consequently from the boiler through f. At
the bottom part of the inner cylinder there are two regulating valves, o and k, one of which, o, either admits the steam to pass from the surrounding cavity g, through a passage i, into the space of the interior cylinder below the pis ton, or shut out the steam from that space at pleasure ; the other valve k, opens or shuts the end of the eduction pipe m, which leads to the condenser e. This condenser is a close vessel, made of thin metal, and furnished with an air pump n, having valves and a bucket e, for ex hausting the air, and drawing off the water produced by the condensation of the steam, along with the air extricated from the water in boiling, and rising with the steam. The air pump is constructed like a common pump, except that it has a lid or cover on the top of the barrel, to keep them pressure of the atmo sphere from bearing constantly upon the bucket. The rod d of the bucket 'wastes through a stuffing-box in the lid, and is suspended by a chain from the great working beam of the engine. The condenser 1, and the air•pump a, are placed in a large cistern of cold water x, which may be supposed to be situated under the floor of the engine-house, between the cylinder and the wall on which the beam is supported, and supplied constantly with fresh cold water from a small pump worked by the engine.
Now, suppose steam be allowed to enter through f into g, between the jacket and the cylinder, and also the upper part of the cylinder above the piston. The condenser i is exhausted of its air by opening both valves o and k, and letting the steam blow through it ; when this is effected, and the valves closed, the external cold in x condenses it and leaves a vacuum in the con denser, whilst the cylinder A is all the while full of steam from the space g, both above and below the piston h h ; the steam-valve o being shut, cuts off all communication with the under side of the piston from the steam in g, or in the boiler ; and at the same time the exhausting-valve k from the condenser is opened, when the steam rushes with great 'violence from the space of the cylinder A, below the piston, through the eduction pipe vs, into the vacuum of the condenser, till it comes in contact with the cold sides of the condenser L The steam becomes immediately deprived of heat and reduced into water, and a vacuum is thus made beneath the piston h. The steam above the piston ceasing to be counteracted by the steam that was below it, presses between the top of the piston and the bottom of the lid e, with its whole elastic force, and causes the piston to descend to the bottom of the cylinder, carrying along with it the beam, and raising the pump-buckets at the other end. '1'he exhausting valve k is then shut, and the steam-valve o opened, which, allowing the steam to enter below the leaves it at liberty to rise ; in which case the superior weight of the pump-rods the piston to the top of the cylinder, ready to commence another stroke.