THE OPERATIVE MECHANIC

of Mr. Smeaton, from which it appears that sails weathered in the Dutch manner produced nearly a maximum effect, but also from the observations of the celebrated Coulomb. This philosopher examined above 50 windmills in the neighbourhood of Lisle, and found that each of them performed nearly the same quantity of work when the wind moved with the velocity of 18 or 20 feet per second, though there were some trifling differences in the inclination of their windshafts, and in the disposition of their sails. From this fact, Coulomb justly concluded that the parts of the machine must have been so disposed as to produce nearly a maximum effect.

In the windmills on which Coulomb's experiments were made, the distance from the extremity of each sail to the centre of the windshaft or principal axis was 33 feet. The sails were rectangular, and their width was a little more than six feet, five of which were formed with cloth stretched upon a frame, and the remaining foot consisted of a very light board. The line which joined the board and the cloth formed, on the side which faced the wind, an angle sensibly concave at the commencement of the sail, which diminished gradually till it vanished at its extremity. Though the surface of the cloth was curved, it may be regarded as composed of right lines pèrpendicular to the arm or whip which carries the frame, the extremities of these lines corresponding with the concave angle formed by the junction of the cloth and the board. Upon this supposition, these right lines at the commencement of the sail, which was distant about six feet from the centre of the windshaft, formed an angle of 60 degrees with the axis or windshaft, and the lines at the extremity of the wing formed an angle increasing from 78 to 84 degrees, according as the inclination of the axis of rotation to the horizon increased from 8 to 15 degrees; or in the mill-wright's terms, the greatest angle of weather was 30 degrees, and the least varied from 12 to 6 degrees, as the inclination of the windshaft varied from 8 to 15 degrees. A pretty distinct idea of the surface of windmill sails may be conveyed 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.

ON HORIZONTAL WINDMILLS.

A VARIETY of opinions have been entertained respecting the relative advantages of horizontal and vertical windmills. Mr. Smeaton gives a decided preference to the latter; but, when he asserts that horizontal windmills have only one-eighth or one-tenth of the power of vertical ones, he certainly forms too low an estimate of their power. Mr. Beatson, on the contrary, who has a patent for the construction of a new horizontal windmill, seems to be prejudiced in their favour, From an impartial investigation, it will probably appear, that the truth lies between these two opposite opinions; but before entering on this discussion, we must first consider the nature and form of horizontal windmills; which we shall do in presenting the reader with a description of the horizontal mill erected at Margate by Captain Hooper.

Fig. 149 is an upright section, and fig. 150 a plan of the building. HH are the side walls of an octagonal building which contains the machinery. These walls are surmounted by a strong timber framing G G, of the same form as the building, and connected at top by cross-framing to support the roof, and also the upper pivot of the main vertical shaft AA, which has three sets of arms, BB, CC, DD, framed upon it at that part which rises above the height of the walls. The arms are strengthened and supported by diagonal braces, and their extremities are bolted to octagonal wood frames, round which the vanes or floats EE are fixed, as seen in outline in fig. 150, so as to form a large wheel, resembling a water-wheel, which is less than the size of the house by about 18 inches all round This space is occupied by a number of vertical boards or blinds F F, turning on pivots at top and bottom, and placed obliquely, so as to overlap each other, and completely shut out the wind, and stop the mill, by forming a close case surrounding the wheel; but they can be moved altogether upon their pivots to allow the wind to blow in the direction of a tangent upon the vanes on one side of the wheel, at the time the other side is completely shaded or defended by the boarding. The position of the blinds is clearly shown at F F, fig. 150. At the lower end of the vertical shaft A A, a large spur-wheel a a is fixed, which gives motion to a pinion c, upon a small vertical axis d, whose upper pivot turns in a bearing bolted to a girder of the floor n. Above the pinion c, a spurwheel e is placed, to give motion to two small pinions f, on the upper ends of the spindles g, of the mill-stone h. Another pinion is situate at the opposite side of the great spur-wheel a a, to give motion to a third pair of mill-stones, which are used when the wind is very strong; and then the wheel turns so quick as not to need the extra wheel e to give the requisite velocity to the stones. The weight of the main vertical shaft is borne by a strong timber b, having a brass box placed on it to receive the lower pivot of the shaft. It is supported at its ends by cross-beams mortised into the upright posts bb, as shown in the plan, fig. 150. A floor or roof II is thrown across the top of the brick-building to protect the machinery from the weather, and to prevent the rain blowing down the opening through which the shaft descends, a broad circular hoop K is fixed to the floor, and is surrounded by another hoop or case L, which is fixed to the arms D D of the wheel. This last is of such a

size, as exactly to go over the hoop K, without touching it when the wheel turns round. By this means, the rain is completely excluded from the upper room M, which serves as a granary, being fitted up with the bins mm, to contain the different sorts of grain which is raised up by the sacktackle. A wheel i is fixed on the main shaft, having cogs projecting from both sides. Those at the under side work into a pinion on the end of the roller K, which is for the purpose of drawing up sacks. Another pinion is situated above the wheel i, which has a roller projecting out over the flapdoors seen at p, in fig. 150, to land the sacks upon. The two pinions m m, fig. 150, are turned by the great wheel a a, and are for giving motion to the dressing and bolting machines, which are placed upon the floor N, but are not shown in the drawing, being exactly similar to the dressing machines used in all flour-mills. The cogs upon the great wheel a are not so broad as the rim itself, leaving a plain rim about three inches broad. This is encompassed by a broad iron hoop, which is made fast at one end to the upright post b; the other being jointed to a strong lever n, to the extreme end of which a purchase o is attached, and the fall is made fast to iron pins on the top of a frame fixed to the ground. This apparatus answers the purpose of the brake or gripe used in common windmills to stop their motion. By pulling the fall of the purchase o, it causes the iron strap to embrace the great wheel, and produces a resistance sufficient to stop the wheel. The mill can be regulated in its motion, or stopped entirely, by opening or shutting the blinds F, which surround the fan-wheel. They are all moved at once by a circular ring of wood situated just beneath the lower ends of the blinds upon the floor II, being connected with each blind by a short iron link. The ring is moved round by a rack and spindle which descend into the mill-room below, for the convenience of the miller.

The mode of bringing the sails back against the wind, which Mr. Beatson invented, is, perhaps, the simplest and best for that end. He makes each sail A I, fig. 151, to consist of six or eight flaps or vanes, A P, b1, b 1, c 2, &c., moving upon hinges represented by the dark lines, AP, b1, c2, &c., so that the lower side b1 of the first flap wraps over the hinge or higher side of the second flap, and so on. When the wind, therefore, acts upon the sail AI, each flap will press upon the hinge of the one immediately below it, and the whole surface of the sail will be exposed to its action. But when the sail A I returns against the wind, the flaps will revolve round upon their hinges, and present only their edges to the wind, as is represented at EG, so that the resistance occasioned by the return of the sail must be greatly diminished, and the motion will be continued by the great superiority of force exerted upon the sails in the position A I. In computing the force of the wind upon the sail A I, and the resistance opposed to it by the edges of the flaps in EG, Mr. Beatson finds, that when the pressure upon the former is 1872 pounds, the resistance opposed by the latter is only about 36 pounds, or part of the whole force; but he neglects the action of the wind upon the arms, CA, &c., and the frames which carry the sails, because they expose the same surface in the position AI, as in the position EG. This omission, however, has a tendency to mislead us in the present case, as we shall now see; for we ought to compare the whole force exerted upon the arms, as well as the sail, with the whole resistance which these arms and the edges of the flaps oppose to the motion of the windmill. By inspecting the figure it will appear, that if the force upon the edges of the flaps, which Mr. Beatson supposed to be 12 in number, amounts to 36 pounds, the force spent upon the bars CD, DG, GF, FE, &c., cannot be less than 60 pounds. Now, since these bars are acted upon with an equal force, when the sails have the position

A I, 1872 + 60 1932 will be the force exerted upon the sail AI and its appendages, while the opposite force upon the bars and edges of the flaps when returning against the wind will be 36 + 60 96 pounds, which is nearly of 1932, instead of as computed by Mr. Beatson. Hence we may see the advantages which will probably arise from using a screen for the returning sail instead of movable flaps, as it will preserve not only the sails, but the arms and the frame which supports it, from the action of the wind.*

Mr. Brewster makes also the following remark on the comparative power of horizontal and vertical windmills. It has been already stated, that Mr. Smeaton rather underrated the former while he maintained that they have only one-eighth or one-tenth the power of the latter. He observes, that when the vanes of a horizontal and a vertical mill are of the same dimensions, the power of the latter is four times that of the former; because, in the first case, only one sail is acted upon at once; while, in the second case, all the four receive the impulse of the wind. This, however, is not strictly true, since the vertical sails are all oblique to the direction of the wind. Let us suppose that the area of each sail is 100 square feet; then the power of the horizontal sail may be called 100 x sin. 70° (which is the common angle of inclination) =88 nearly; but since there are four vertical sails, the power of them all will be 4 x 88 352: so that the power of the horizontal sail is to that of the four vertical ones as 1 to 3.52, and not as 1 to 4, according to Mr. Smeaton. But Mr. Smeaton also observes, that if we consider the further disadvantages which arises from the difficulty of getting the sails back against the wind, we need not wonder if horizontal windmills have only about or of the common sort. We have already seen that the resistance occasioned by the return of the sails amounts to of the whole force which they received; by subtracting therefore, from, we shall find that the power of horizontal windmills is only, or little more than one-fourth less than that of vertical ones. This calculation proceeds upon a supposition that the whole force exerted upon vertical sails is employed in turning them round the axis of motion; whereas a considerable part of this force is lost in pressing the pivot of the axis or windshaft against its gudgeon. Mr. Smeaton has overlooked this circumstance, otherwise he could never have maintained that the power of four vertical

* The sails of horizontal windmills are sometimes fixed like float-boards on the circumference of a large drum or cylinder. These sails move upon hinges so as to stand at right angles to the drum, when they are to receive the impulse of the wind; and when they return against it, they fold down upon its circumference.

sails was quadruple the power of one horizontal sail, the dimensions of each being the same. Taking this circumstance into the account, we cannot be far wrong in saying that, in theory at least, if not in practice, the power of a horizontal windmill is about one-third or one-fourth of the power of a vertical one, when the quantity of surface and the form of the sails are the same, and when all the parts of the horizontal sails have the same distance from the axis of motion as the corresponding parts of the vertical sails. But if the horizontal sails have the position A I, E G, in fig. 151, instead of the position CA dm, CD on, their effect will be greatly increased, though the quantity of surface is the same; because the part CP3 m being transferred to BI3 d, has much more power to turn the sails. Having this method, therefore, of increasing the effect of horizontal sails, which cannot be applied to vertical ones, we would encourage every attempt to improve their construction, as not only laudable in itself, but calculated to be of essential utility in a commercial country.-See Dr. Brewster's valuable Appendix to Ferguson's Lectures.

FLOUR-MILLS.

IN fig. 152 we have given a section of a double flour-mill, reduced from Gray's Experienced Mill-wright, with the following account:

A A, the water-wheel. RB, its shaft or axle. CC, a wheel fixed upon the same shaft, containing 90 teeth or cogs, to drive the pinion No. 1, having 23 teeth, which is fastened upon the vertical shaft D. No. 2, a wheel fixed upon the shaft D, containing 82 teeth, to turn the two pinions FF, having 15 teeth, which are fastened upon the iron axles or spindles that carry the two upper mill-stones. EE, the beam or sill that supports the frame on which the under mill-stones are laid. GG, the cases or boxes that enclose the upper mill-stones; they should be about two inches distant from the stone all round its circumference. TT, the bearers, called bridges, upon which the under end of the iron spindles turn. These spindles pass upwards through a hole in the middle of the nether mill-stones, in which is fixed a wooden bush that their upper ends turn in. The top part of the spindles, above each wooden bush, is made square, and goes into a square hole in an iron cross, which is admitted into grooves in the middle and under surface of the upper mill-stone. By this means that stone is carried round along with the trundles FF, when turned by the wheel No. 2. One end of the bridges TT is put into mortises in fixed bearers; and the other end into mortises in the bearers that move at one end on iron bolts, their other ends hanging by iron rods having screwed nuts, as UU; so that when turned forward or backward they raise or depress the upper mill-stones, according as the miller finds it necessary. SS, the feeders, in the under end of each of which is a square socket that goes upon the square of