air upon the depression of the spirits; on the other hand, when the lever QR falls, and the cup is at rest, the valve at S closes the tube, and prevents the spirits from being wasted by evaporation.

It is lastly to be remarked, that both the sensibility and the range of the instrument may be infinitely increased; for, on the one hand, by enlarging the proportion between the diameters of the wide and narrow parts of the tube, we enlarge in a much higher proportion the extent of scale corresponding to any given variation of velocity; and on the other hand, by deepening the cup so as to admit when it is at rest a greater height of mercury above the lower end of the tube, we lengthen the column of spirits which the mercury can support, and consequently enlarge the velocity, which, with any given sensibility of the instrument, is requisite to depress the spirits to the bottom of the scale. Hence the tachometer is capable of being employed in very delicate philosophical experiments, more especially as a scale might be applied to it, indicating equal increments of velocity. But in the present account it is merely intended to state how it may be adapted to detect in machinery every deviation from the most advantageous movement.

General Observations.-In setting out the geering of a mill, it should be the object of the engineer to place the heaviest machinery nearest the moving power, as, in transmitting motion to a great distance, not only the weight of shafting is to be taken into consideration, but the friction which exists in all the different bearings, and which is greatly increased by a small obstacle placed beyond those bearings.

Care likewise should be taken to make as few bearings as possible, still keeping in view that the shafts must not be allowed to swag. Rules might be given for the distances of the bearings of the shafting, if the shafting had only to move itself, but having to carry various sized pullies, both their weight and the weight of the machinery they turn must be taken into consideration, which compel us to forego the attempt; it is, however, necessary to state, that it is better to have a bearing too many than to allow a shaft to bend, as it cannot then run true in its steps or journals.

In forming couplings, great care should be taken to make them fit, so that the coupled shaft may move as though of the same piece with the driving shaft: nor can simplicity be too strongly recommended, that the coupled shaft may, in case of an accident, be instantaneously disengaged, for the

loss of time arising from any accident is of serious importance to the manufacturer. Couplings should be placed near the bearings, as there is there the least swag, and the shaft is of course the weakest at the couplings. The same observation is applicable to the disposing of wheels and pullies.

Pullies have been sometimes formed in two halves for putting upon the shaft without taking the shaft down, but their adoption is by no means general, as there is some difficulty in fixing them true whilst the shaft is in its place.

Straps to drive geering should be avoided whenever wheels can be substituted, as they are very liable to stretch and break, and do not transmit regular motion. In fixing the wheels and pullies upon a shaft, which is mostly done by driving wedges in the bush of the wheel or pulley, called staking them on, great pains should be taken to have them true, which can only be done by driving the wedges regularly on each side to the same degree of tightness. It most generally happens when one wedge is over-driven, the workmen, rather than take the trouble to alter it, will let it remain; but this is of more importance than is generally imagined, for if a wheel is not true, it cannot work in the pitch line, all round, and where it is out it will shake, or have, what is called, back-lash, which, happening always in the same place, will wear the wheels irregularly. If a pulley is not true, it will communicate irregular motion by its strap, and likewise cause ar. irregular stress upon the shaft on which it works, much to the detriment of the bearing.

Chains have been beneficially introduced as substitutes for straps in driving heavy geer.

Shafts should be circular, as they are less likely to catch any thing, and have a much neater appearance. The same may be said of couplings. The wheels of the geering should be always enclosed in a casing of wood, called boxing off, to prevent any thing falling in between them, or accidents occurring to the people who may be working near them. The wheels should be furnished with brushes resting upon their faces, to distribute the grease equally and to keep it between the teeth and on starting a new pair of wheels, a little emery may be put on with the grease, to bring them to a smooth face.

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The following general observations on the construction of MACHINES, and on the regulating of their motions, appear to be highly worthy of the Mill-wright's attention; we have, therefore, extracted them from Dr. Robison's articlę

on Machinery, inserted in the Supplement to the Encyclo pædia Britannica.

When heavy stampers are to be raised, in order to drop on the matters to be pounded, the wipers by which they are lifted should be made of such a form, that the stamper may be used by a uniform pressure, or with a motion almost perfectly uniform. If this is not attended to, and the wiper is only a pin sticking out from the axis, the stamper is forced into motion at once. This occasions violent jolts to the machines, and great strains on its moving parts and their points of support; whereas, when they are gradually lifted, the inequality of desultory motion is never felt at the impelled point of the machine. We have seen pistons moved by means of a double rack on the piston rod. A half wheel takes hold of one rack, and raises it to the required height. The moment the half wheel has quitted that side of the rack, it lays hold of the other side, and forces the piston down again. This is proposed as a great improvement; connecting the unequable motion of the piston moved in the common way by a crank. But it is far inferior to the crank motion. It occasions such abrupt changes of motion, that the machine is shaken by jolts. Indeed, if the movement were actually executed, the machine would be shaken to pieces, if the parts did not give way by bending and yielding. Accordingly, we have always observed that this motion soon failed, and was changed for one that was more smooth. A judicious engineer will avoid all such sudden changes of motion, especially in any ponderous part of a machine.

When several stampers, pistons, or other reciprocal movers, are to be raised and depressed, common sense teaches us to distribute their times of action in a uniform manner, so that the machine may always be equally loaded with work. When this is done, and the observations in the preceding paragraph attended to, the machine may be made to move almost as smoothly as if there were no reciprocations in it. Nothing shows the ingenuity more than the artful yet simple and effectual contrivances for obviating those difficulties that unavoidably arise from the very nature of the work that must be performed by the machine, and of the power employed.

There is also great room for ingenuity and good choice in the management of the moving power, when it is such as cannot immediately produce the kind of motion required for effecting the purpose. We mentioned the conversion of the

continued rotation of an axis into the reciprocating motion of a piston, and the improvement which was thought to have been made on the common and obvious contrivance of a crank, by substituting a double rack on the piston-rod, and the inconvenience arising from the jolts occasioned by this change. We have seen a great forge, where the engineer, in order to avoid the same inconvenience arising from the abrupt motion given to the great sledge hammer of seven hundred weight, resisting with a five-fold momentum, formed the wipers into spirals, which communicated motion to the hammer almost without any jolt whatever; but the result was, that the hammer rose no higher than it had been raised in contact with the wiper, and then fell on the iron bloom with very little effect. The cause of its inefficiency was not guessed at; but it was removed, and wipers of the common form were put in place of the spirals. In this operation, the rapid motion of the hammer is absolutely necessary. It is not enough to lift it up; it must be tossed up, so as to fly higher than the wiper lifts it, and to strike with great force the strong oaken spring which is placed in its way. It compresses this spring, and is reflected by it with a considerable velocity, so as to hit the iron as if it had fallen from a great height. Had it been allowed to fly to that height, it would have fallen upon the iron with somewhat more force, (because no oaken spring is perfectly elastic,) but this would have required more than twice the time.

In employing a power which of necessity reciprocates, to drive machinery which requires a continuous motion (as in applying the steam-engine to a cotton or grist mill,) there also occur great difficulties. The necessity of reciprocation in the first mover wastes much power; because the instrument which communicates such an enormous force must be extremely strong, and be well supported. The impelling power is wasted in imparting, and afterwards destroying, a vast quantity of motion in the working beam. The skilful engineer will attend to this, and do his utmost to procure the necessary strength of this first mover, without making it a vast load of inert matter. He will also remark, that all the strains on it, and on its supports, are changing their directions in every stroke. This requires particular attention to the manner of supporting it. If we observe the steamengines which have been long erected, we see that they have uniformly shaken the building to pieces. This has been owing to the ignorance or inattention of the engineer in this particular. They are much more judiciously erected now,

experience having taught the most ignorant that no building can withstand their desultory and opposite jolts, and that the great movements must be supported by a frame-work independent of the building of masonry which contains it.*

The engineer will also remark, that when a single-stroke steam-engine is made to turn a mill, all the communications of motion change the direction of their pressure twice every stroke. During the working stroke of the beam, one side of the teeth of the intervening wheels is pressing the machinery forward; but during the returning stroke, the machinery, already in motion, is dragging the beam, and the wheels are acting with the other side of the teeth. This occasions a rattling at every change, and makes it proper to fashion both sides of the teeth with the same care.

It will frequently conduce to the good performance of an engine, to make the action of the resisting work unequable, accommodated to the inequalities of the impelling power. This will produce a more uniform motion in machines in which the momentum of inertia is inconsiderable. There are some beautiful specimens of this kind of adjustment in the mechanism of animal bodies.

It is very customary to add what is called a fly to machines. This is a heavy disk or hoop, or other mass of matter balanced on its axis, and so connected with the machinery as to turn briskly round with it. This may be done with the view of rendering the motion of the whole more regular, notwithstanding unavoidable inequalities of the accelerating forces, or of the resistances occasioned by the work. It becomes a regulator. Suppose the resistance extremely unequal, and the impelling power perfectly constant; as when a bucket-wheel is employed to one pump. When the piston has ended its working stroke, and while it is going down the barrel, the power of the wheel being scarcely opposed, it accelerates the whole machine, and the piston arrives at the bottom of the barrel with a considerable velocity. But in the rising again, the wheel is opposed by the column of water now pressing on the piston. This immediately retards the wheel; and when the piston has reached

* The gudgeons of a water-wheel should never rest on the wall of the building. It shakes it; and if set up soon after the building has been erected, it prevents the mortar from taking firm bond; perhaps by shattering the calcareous crystals as they form. When the engineer is obliged to rest the gudgeons in this way, they should be supported by a block of oak laid a little hollow. This softens all tremors, like springs of a wheel carriage. This practice would be very serviceable in many other parts of the construction.