shutting of this valve, and consequently the divergence or convergence of the balls, or the velocity of the shaft, will be the quantity of steam admitted to the cylinder. The governor is frequently applied to the water wheel, and acts in a similar way by a board or valve in the shuttle, which delivers the water to the wheel. So likewise in the windmill, it is employed to furl or unfurl more or less sail.

Sometimes the governor is found inadequate to the regulation of the machine, and another contrivance of great power and simplicity is introduced. The machine is made. to work a pump, which tends continually to fill a cistern with water. From this cistern there proceeds an eduction pipe, leading to the reservoir, from which the water is drawn by the pump. This simple contrivance is so regulated, that when the machine goes with its proper velocity, the pump throws just as much water into the cistern as the ejection pipe draws from it; consequently, the water in the cistern remains at the same level. But if the machine goes too fast, then the pump will throw in more water than is let out by the ejection pipe, wherefore the level of the water will rise in the cistern. If the machine goes too slow, the level of the water will in like manner fall. Now, on the surface of the water in the cistern, there is a float which rises or falls with the surface of the water; and is thus made to answer the same purpose as the ring of the governor. It may be observed, that the delicacy of this kind of regulator will depend, in a great measure, upon the smallness of the surface of the water which supports the float; for then a small difference between the supply and discharge, will cause a greater difference in the elevation or depression of the surface, than if the surface were large.

To procure a constant supply of steam in the steam engine, it is necessary that the water in the boiler be always at the same level. To effect this purpose, there is a lever fixed on a support, on the top of the boiler, to one end of which lever there is attached a slender rod, which descends into the boiler, and is terminated by a float, which rests on the surface of the water in the boiler. To the other end of the lever, there is attached another rod, to the end of which is affixed a valve, opening and shutting the orifice of a pipe which leads into the boiler. The top of the pipe, where the valve is placed, opens into a cistern of water, which is supplied by a pump driven by the engine itself.

Wher. the water in the boiler falls below its common level in consequence of the formation of steam, the float falls with it, and consequently depresses that side of the lever to which the float rod is attached; the other arm rises and opens the valve at the top of the pipe, which leads from the cistern into the boiler, and thus admits water until the float rises to the proper height, and then the valve is closed. In this beautiful contrivance, the water is not supplied to the boiler in jolts, but the float and valve continuing in a state of constant and quick vibration, the supply is rendered quite constant.

There is a very ingenious contrivance called the Tachometer, from its use as a measure of small variations in velocity, which is often employed in the steam engine and other machinery. The simplicity of this contrivance will render its action easily understood. If a cup with any fluid, as mercury, be placed on a spindle, so that the brim of the cup shall revolve horizontally round its centre, then the mercury in the cup will assume a concave form, that is, the mercury will rise on the sides of the cup, and be depressed in the middle; and the more rapid the motion of the cup is, the more will the surface of the mercury differ from a plane. Now, if the mouth of this cup be closed, and a tube inserted in it, terminated in the cup by a ball-shaped end, and half filled with some coloured fluid, as spirits of wine and dragon's blood; then it is clear, that the more the surface of the mercury is depressed, the more the fluid in the tube will fall, and vice versa: consequently, the rapidity or slowness of the motion of the cup, will be indicated by the height of the coloured fluid in the tube; and thus it becomes a measure of small variations in velocity.

In the steam engine, we also find an apparatus for regulating the strength of the fire of the boiler, which apparatus is called the self-acting damper. There is a tube inserted into the boiler, reaching nearly to the bottom, which tube is open at both ends. Now, from the principles of Pneumatics, it is plain, that the greater the pressure of the steam in the boiler is, the water will be pressed to the greater height in this tube. The water in the tube supports a weight, to which there is attached a chain going over two wheels; and to the other end of the chain is attached a plate, which slides over the mouth of the flue which leads into the fire. These things are so formed, that the rising of the weight

in the tube will cause more or less of the flue to be covered by the plate; and thus increase or diminish the quantity of air which feeds the fire. Now, if there is too much steam produced, there will be a greater pressure on the surface of the water in the boiler, and it will be forced up the tubethe weight in the tube will be raised, and consequently the plate at the other end of the chain will fall, and cover more of the mouth of the flue, and thus diminish the quantity of air which feeds the fire; and there will consequently be generated in the boiler a less quantity of steam.

We come now to speak of the nature and use of the fly wheel. A fly in mechanics may be defined to be a heavy wheel or cylinder, which moves rapidly upon its axis, and is applied to a machine for the purpose of regulating its motion.

We have already stated that there are many circumstances which tend to render the motion of a machine irregular-variation in the energy of the first mover, whether it be water, wind, steam, or animal strength-variation in the resistance or work to be done-and variations in the efficacy of the machine itself, arising from the nature of its construction, whereby it is of necessity more effective in one position than in another. We have already seen how many of these irregularities are compensated, and we are now come to speak of the fly, which is the simplest and most effective of them all. The principle on which the fly acts is that of inertia, one of the most important of the first principles of mechanical science. At any one given time, a body must be in one or other of these two states-rest or motion. And let any body be in one or other of these two states, it has no power within itself to change it,—if it be at rest, it has no power to put itself in motion-and if in motion, it has no power in itself either to increase, diminish, or destroy that motion. From a knowledge of this fact, and from what was stated before on the momentum, or moving force of a body, that it is the quantity of matter multiplied by the velocity of the moving body-the nature of the operation of the fly will be easily understood.

As the fly wheel, to do. its office effectually, must have a considerable velocity, it is clear that its rim, which has a considerable weight, must also have a considerable momentum, and consequently a considerable power to overcome any tendency either to increase or retard its motion.

To apply these observations to actual cases, let us sup pose that a single horse drives a gin. When the gin has been set in motion, the animal cannot exert a uniform strengththere will be occasional increases and relaxations in the velocity of the gin; but suppose a fly wheel to be added, then, whenever the animal slackened its exertions, the machine would have a tendency to move slower, but the momentum which the fly had acquired, would overcome this tendency to retardation, and continue the motion of the machine at the same rate as before, until the animal had recovered itself so as to exert the same strength as before. So, likewise, if the animal exerted an extraordinary pull, the inertia of the wheel would oppose a resistance which would check the tendency to increase in the velocity of the gin. In this way the fly wheel regulates the motion of the gin, whether the animal takes occasional rests, or makes occasional extraordinary exertions. It is evident that the fly would operate in the same way, if the first mover were steam, water, or wind, and that the other regulators which we have described, are merely assistants to the fly wheel.

Variations in the resistance, or work to be performed, are also compensated by the fly wheel. For instance, in a small thrashing mill without a fly. When the machine is not regularly fed with the corn, there will be an occasional resistance, which will have a sensible effect on the whole train of the machinery, even the water wheel itself; which irregularity, may, however, be avoided by the introduction of a fly, as its inertia will procure equality of motion: but it may be observed, that when the machine is large, there will be less necessity for a fly, as the inertia of the machine itself will then effect the same purposes.

It was before stated, that even supposing the first mover and resistance to be perfectly uniform, the machine itself is liable to variations in energy at different positions. It was seen, for instance, that a crank is more effective in one position than another; but the momentum communicated to the fly, when the crank is in the most effective position, will carry the crank past its least effective position. There are many cases, however, where there are irregularities of motion proceeding from the nature of the machinery, which could be compensated better than with a fly. Thus, if a bucket is to be drawn from the bottom of a coal pit, which is 60 fathoms in depth: the weight of bucket being 14

rwt., and the chain by which it is coiled up round the cylin der weighing 8 lbs. to every fathom,-it is plain, that when the bucket is at the bottom, not only the weight of the bucket, but also the weight of the chain, will require to be overcome in the raising of the bucket. Now the weight of the chain is 60 + 8 480 lbs., and the amount of the weight of the bucket is 14 cwt. or 1568 lbs.; hence 1568 + 480 = 2048 lbs. ; but the weight of the chain will always be getting less as it is coiled round the cylinder, until the bucket comes to the cylinder, when the chain will be all coiled, and there will remain only the weight of the bucket, Now, the use of a fly may be advantageously dispensed with, if the barrel on which the chain is coiled is formed like a cone; the diameter of the barrel thus increasing with the uniform diminution of the weight.

The effect of the fly wheel in accumulating force, has led some to suppose that there is, positively, a creation of force in the fly; but this is a mistake, for it is only, as it were, a magazine of power, where there is no force but what has been delivered to it. The great use of the fly wheel is thus to deliver out at proper intervals, that force which has been previously communicated to it; and although there is absolutely a small loss of power by the use of the fly, yet this is more than compensated by its utility as a regulator.

The motion of machines may, as stated before, be re duced to three kinds. That which is gradually accelerated, which generally takes place at the commencement of a machine's action: that which is entirely uniform: that which is alternately accelerated and retarded. The nearer that the motion of a machine approaches to uniformity, the greater will be the quantity of work done.

In order that the few remarks, which we intend to make on the effect of machines, may be clearly understood, we desire the reader to attend to the following definitions.

The impelled point of any machine, is that point at which the force which moves the machine, may be considered as applied as the piston of a steam engine, or the float board of a water wheel.

The working point, on the contrary, is that point where the resistance may be supposed to act.

The velocity of the moving power is the same as the velocity of the impelled point, the velocity of the resistance is the same as the velocity of the working point.