accurately known to the weighers by mere inspection; and that the book-keeper has it in his power, with a glance, to discover whether the weighers call the proper weight; which is impracticable by the present modes of weighing. Boards for weighing smaller quantities than a ton might be made on the same principle, for weights of the same dimensions, with scales adapted to the size of the board. It is to be understood that the weighing is performed without "striking the weights," which is the common phrase for lifting all the weights off the board each operation: therefore an appropriate mode, according to situation and circumstances, must be adopted to support the board with the weights, while the package weighed is removing from its board to give place to another; when, in some instances, the largeness of the package bulges out the ropes of the board, rendering it necessary to raise the board with the weights a little higher. In some cases the prop, fig. 7., will answer the pose, the pinion being moved by a winch. In other cases the lever, fig. 8., might be adopted, and in particular instances, the whole beam and scales, with the goods and weights, might be raised and lowered by the lever, fig. 9., assisted by the wheel and pinion.

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The greatest individual weight, for the purpose of being portable, is a half-hundred weight. The common balance is used with this weighing apparatus, as it proves to be the best kind of balance known; being more true for very ponderous bodies than the steelyard, which is sometimes used where great accuracy is not required. When a very light package is to be weighed with a board adapted for a much greater weight, a hook and eye are to be used at each of the two cords, suspending the board for the weights at A and c, in order to shortenthem and prevent the board from leaning to one side. Where a chain instead of a rope is used, one of its links will serve as an eye to the hook.

WHEELS acting upon each other are the instruments by which the transmission of mechanic force from one part of a system of machinery to another is commonly and conveniently effected. The due connexion of the moving parts is accomplished, either by the mutual action of proper formed teeth (see the article TEETH in this volume), by straps or endless bands, or by the friction of one face of a wheel against another. The latter method has, when adopted, been generally in small light works, where the pressure upon the different parts of the machinery is never considerable.. Mr. Nicholson saw a drawing of a spinning-wheel for children, at a charity school, in which a large horizontal wheel with a slip of buff-leather glued on its upper surface, near the outer edge, drove 12 spindles, at which

the same number of children sat. The spindles had each a small roller likewise faced with leather, and were capable, by an easy and instantaneous motion, of being thrown into contact with the large wheel at pleasure. Each child, therefore, could throw her own part of the apparatus into work, or cause it to stop, as often or as long as she pleased. The winding bobbins for yarn at the cotton-mills operate on the same simple and elegant principle, which possesses the advantage of drawing the thread with an equal velocity, whatever may be the quantity on the bobbin, and cannot break it. We are not aware that the same mode of communication has been adopted in large works, except in a saw-mill, by Mr. Taylor, of Southampton. In this the wheels act upon each other by the contact of the end grain of wood instead of cogs: the whole makes very little noise, and wears very well: it has now been in use nearly twenty years. There is, of consequence, a contrivance to make the wheels bear firm against each other, either by wedges at the sockets, or by levers. This principle and method of transmitting mechanic power certainly deserves every attention; particularly as the customary mode by means of teeth requires much skill and care in the execution; and, after all, wants frequent repairs.

WIND-MILL, as its name imports, a mill for any purpose which receives its motion from the impulse of the wind.

The internal structure of wind-mills are, of course, much the same as those of water-mills: the difference between them lying chiefly in the exterior apparatus, the one to receive the force of the water, the other that of the wind. The external apparatus in a wind-mill consists chiefly of the sails or vanes, which are commonly four, placed in nearly a vertical position, and as they turn giving a rotatory motion to an axis inclining but a little from the horizon. The usual construction and appearance of the sails is too well known to need any minute description; though it may be expected that we shall treat a little of the method of weathering the sails, &c. Now a pretty distinct idea of the surface of wind-mill sails may be obtained by conceiving a number of triangles standing perpendicular to the horizon, in which the angle contained between the hypothenuse and the base is constantly diminishing: the hypothenuse of each triangle will then be in the superficies of the vane, and they would form that superficies if their number were infinite.

Mr. Richard Hall Gower, a gentleman in the sea service of the East India Company, made some judicious experiments with a view of determining the proper angles of weather which ought to be given to the vanes of a vertical wind-mill: his general conclusion is, that each vane should be a spiral generated by the

circular motion of a radius, and of a line moving at right angles to the plane of the circular motion. The construction he deduces from his enquiries is simple, being this: The length, breadth, and angle of weather at the extremity of a vane being given; to determine the angles of weather at different distances from the centre.

Let AB, fig. 9. pl. XXXV. be the length of the vane; BC its breadth: and BCD the angle of weather at the extremity of the vane, equal to 20 degrees. With the length of the vane AB, and breadth BC, construct the isosceles triangle ABC: from the point B draw BD perpendicular to CB, then BD is the proper depth of the vane.

Divide the line AB into any number of parts (five, for instance); at those divisions draw the lines 1E, 2F, 3G, and 4н, parallel to the line BC; also from the points of division 1, 2, 3, and 4, draw the lines 11, 2K, 3L, and 4м, perpendicular to 1E, 2F, 3G, &c. all of them equal in length to BD. Join El, FK, GL, and HM: then the angles IEI, 2FK, 3GL, and 4HM, are the angles of weather at those divisions of the vane; and if the triangles be conceived to stand perpendicular with the plane of the paper, the angles I, K, L, M, and D, becoming the vertical angles, the hypothenuse of these triangles will, as before suggested, give a perfect idea of the weathering of the vane as it recedes from the centre. (Phil. Mag. No. 14.)

Some theoretical remarks on this subject are inserted in vol.i. art. 547.

As the direction of the wind is very uncertain, it becomes necessary to have some contrivance for turning the sails towards it, in order to receive its force in whatever way it may turn; and for this purpose two general methods are in use. In the one, the whole machine is sustained upon a moveable arbour or axis, perpendicular to the horizon, which is supported by a strong stand or foot very firmly fixed in the earth; and thus by means of a lever the whole machine may be turned round as occasion requires. In the other method, only the roof, which is circular, can be turned round by means of a lever and rollers, upon which the circular roof moves. This last kind of wind-mill is mostly built of stone, in the form of a round turret, having a large wooden ring on the top of it, above which the roof, which must likewise be of wood, moves upon rollers, as has been already mentioned. To effect this motion the more easily, the wooden ring which lies on the top of the building is furnished with a groove, at the bottom of which are placed a number of brass truckles at certain distances, and within the groove is placed another ring, by which the whole roof is supported. Beams

are connected with the moveable ring, and a rope is fastened to one of them, which at the lower extremity is fitted to a windlass or axis in peritrochio; and this rope being drawn through an iron hook fixed at the ground and the windlass turned round, the sails and roof will be turned round also, in order to catch the wind in any direction. Both these methods of construction have their advantages and disadvantages. The former is the least expensive, as the whole may be made of wood, and of any form that is thought proper; while the other requires a more costly building: and the roof being round, the building must also be so, while the former can be made of any form, but has the inconvenience of being liable to be carried off altoge ther by a very high wind. As both these methods of adjusting the windshaft require human assistance, it would be very desirable that the same effect should be produced by the action of the wind solely. This may be done by fixing a large wooden vane or weather-cock at the extremity of a long horizontal arm which lies in the same vertical plane with the windshaft. By this means when the surface of the vane and its distance from the axis of motion have sufficient magnitude, even a gentle breeze will so act upon this vane as to turn the machinery, and move the sail and windshaft to their proper position. This expedient may be adopted whether the mill has a moveable roof or revolves upon a vertical shaft.

In art. 50. of the Introductory part of this volume, we have stated the principal results of the experiments and researches of Smeaton, relative to the shape, position, and magnitude of sails, when four is the number adopted. To these it might be proper to add here, some of the remarks which have been made by Parent, Euler, and other philosophers: but as none of them, except a few by Coulomb, appear any way comparable in point of practical utility with those of Smeaton, and as they include, besides, some very intricate investigations, we conceive they may be omitted without any serious disadvantage to the student.

We shall now, therefore, proceed to describe a wind-mill, varying in many respects from the common construction. This mill was invented by Mr. James Verrier, of North Curry, in Somersetshire, who received a premium from the Society of Arts, for this useful specimen of his ingenuity. Mr. Verrier has contrived a register or regulator, by which the vanes are suffered to yield and give way to the impetus of the wind, when it is too forcible; and when it is too languid, it brings the vanes up to the wind, till its force is sufficient to give the mill a proper degree of velocity: by this contrivance, the wind is justly proportioned to the resistance or number of stones put to

work, and the mill less liable to be set on fire, or destroyed by the violence of its motion. The vertical shaft of this mill is also much shorter than usual, in consequence of which the whole building (and especially the floor on which the stones are placed) is considerably stronger, and less liable to vibrate than in the common mills. The substance of the following description of Mr. Verrier's mill is given in Bailey's account of Machines, approved by the Society of Arts, vol. ii. p. 47., the edit.

of 1782.

This mill which has eight quadrangular sails, is represented in fig. 7. pl. XXXVII. where AAA are the three principal posts, 20 feet, 7 inches long, 22 inches broad at their lower extremities, 18 inches at their upper, and 17 inches thick. The column B is 12 feet, 10 inches long, 19 inches in diameter at its lower extremity, and 16 inches at its upper: it is fixed in the centre of the mill, passes through the first floor E, having its upper end secured by the rails GG. EEE are the girders of the first floor, one of which only is seen, being 8 feet 3 inches long, 11 inches broad, and 9 thick: they are mortised into the principal posts AAA and the column B, and are about 8 feet 3 inches above the ground floor. DDD are three posts, 6 feet 4. inches long, 9 inches broad, and 6 inches thick: they are mortised into the girders EF of the first and second floor, 2 feet 4 inches distant from the posts A, &c. FFF are the girders of the second floor, 6 feet long, 11 inches broad, and 9 thick: they are mortised into the posts A, &c. and rest upon the upper ends of the posts D, &c. The three rails GGG are 3 feet 1 inches long, 7 inches broad, and 3 thick: they are mortised into the posts D and the upper end of the column B, 4 feet 3 inches above the floor to their upper edges. P is one of the arms which support the extremities of the bray-trees: its length is 2 feet 4 inches, its breadth 8 inches, and its thickness 6 inches. 1 is one of the bray-trees into which the extremity of one of the -bridge-trees K is mortised. Each bray-tree is 4 feet 9 inches long, 9 inches broad, and 7 thick and each bridge-tree is 4 feet 6 inches long, 9 inches broad, and 7 thick, being curved 9 inches from a right line, and furnished with a piece of brass on its upper surface to receive the under pivot of the millstones. LL are two iron screw bolts which raise or depress the fore-ends of the bray-trees. MMM are the three millstones, and NNN the iron spindles each 9 feet long on which the upper millstones are fixed. o is one of three wallowers which are fixed on the upper ends of the spindles NNN: they are 16 inches in diameter, and each is furnished with 14 trundles. f is one of the carriagerails in which the upper pivot of the spindle turns, and is 4 feet 2 inches long, 7 inches broad, and 4 thick. It turns on an iron