usually experienced in viewing the successful invention of another person.

There is yet one other contrivance of this great man which must be explained - the governor; and to make this explanation useful, a few preliminary remarks are required.

The best means of obtaining a rotatory motion, as already observed, is by the use of a crank. But in the revolution of a crank there are two dead points, or, in other words, points where the moving power ceases to impel it. These are at the highest elevation and lowest depression, when on the verge of changing the direction of its motion, which is when the piston is at the top and bottom of the cylinder. If the crank were at these points to come absolutely to rest, the steam would be unable to move the engine; but the momentum acquired by the first stroke, which is made before any great load comes upon it, carries it over these positions. From this cause, however, there will always be an irregularity of motion, and it may happen, when certain valves do not act promptly, and an excessive load is suddenly put on to the engine, that the crank may be brought to rest on a dead point. To correct this irregularity, Watt placed a heavy metal wheel, called a flywheel, upon the shaft. Being put into motion with the shaft, its momentum assists in carrying it over the positions where the steam ceases to When the crank is in those positions where the power is most effective upon it, a portion of that power is expended in increasing

act.

the velocity of the fly-wheel, and when in those where the power is lost, it is carried forward by the fly-wheel.

When these arrangements have been made in the best and most effective manner, the engine will still be wanting in regularity of motion. If the amount of steam supplied to the cylinder were always the same, there would be a constant increase and decrease of velocity from a perpetual change in the load. In an engineer's shop, for example, all the machines are rarely at work at the same moment-it is but seldom that the lathes, punches, shears, and other tools, are in operation at the same time. Upon the increase of the load which it has to bear, the engine must have a diminished velocity, and when it is greatly lessened or altogether removed, the speed will be undesirably, or even dangerously increased. To regulate the speed according to the load, there must be some means of decreasing or increasing the proportion of steam admitted into the cylinder, as the change of load may require. For this purpose Watt fixed in the steam pipe a valve of peculiar construction, called a throttle valve. We have given a section of this contrivance as it appears when fixed in the steam pipe, and partly open. It consists of a thin circular plate of brass, (iron is not a suitable metal, as it would soon be destroyed by oxidization,) so mounted on an axis that it may either be made to present its edge to the current of steam, and so leave the passage open, entirely close it, or take any

intermediate position. For the regulation of this valve a small lever was, in the first instance,

provided, so as to enable the engine driver to adjust at pleasure the quantity of steam admitted to the cylinder. But Watt soon perceived that he could not leave the control of the throttle valve to the driver, however skilful and vigilant he might be. There is a source of irregularity in the motion of the engine, independent of the load. The elastic force of the steam is liable to rapid change from the slackness or the intensity of the fire, and from many other sources of disturbance, and the engineer, if he perform his duty, finds ample employment in other necessary labours. From these considerations, and, perhaps, from a desire to make the engine selfacting, the idea of leaving the throttle valve to such precarious attendance was abandoned. A mechanical arrangement being imperative, Watt adopted the best that could be found, and one

that will probably remain in use as long as the steam engine itself.

The governor of a stationary engine is one of the first things that attracts the attention of a stranger in the engine room, and it is certainly not the less interesting when the principle of its action is investigated. Two balls, which are so connected with the fly-wheel shaft as to partake in all its changes of velocity, are seen to revolve round and round, faster or slower, collapsing or separating, in a strange, and at first in a very unaccountable manner. It would scarcely be imagined that the motion of these two balls towards or from each other regulates the admission of steam into the cylinder, and yet such is the fact. They are connected by a series of levers with the throttle valve,-when diverging, they close; when collapsing, they open

it. Whatever alteration there may be in the velocity of the fly-wheel shaft, it is, as we have already stated, communicated to the balls. Now it is generally known that the greater the velocity of any body revolving round a centre, the greater is the centrifugal or centre flying force, and, therefore, just in proportion to the speed of the fly-wheel shaft will be the distance to which these balls separate. The effect of this, communicated by a series of levers, is, partially or entirely to close the steam passage. When the balls are close together, the velocity is small, and the steam passage is quite open, because the valve presents its edge in the direction of the passage of the steam. As the

velocity increases, the valve is turned more and more, until at last it completely covers the opening, and shuts off the further admission of steam into the cylinder. The governor has thus a complete control of the throttle valve, and every irregularity of speed is immediately corrected by the admission of a greater or less quantity

of steam.

In the year 1800, the patent right granted to Bolton and Watt expired, and the great mechanist then retired from business, leaving the works at Soho to Mr. Bolton, the son of his partner, and his own two sons, James and Gregory. Gregory Watt died four years after of a pulmonary complaint, in the twenty-seventh year of his age, an event deeply afflicting to his father. James Watt continued for some time to carry on the works at Soho, but he also is now dead; and not long since the auctioneer stood among a crowd of curious purchasers in this once busy and prosperous workshop.