An undershot mill, with oblique float boards, was invented by the late Mr. Besant of Brompton ; it promises to be of great service in some situations. In com mon water-wheels, more than half the quantity of that fluid passes from the gate through the wheel without affording it any assistance : the action of the floats is resisted by the incumbent atmosphere, at the moment when these leave the tail. water ; and as a similar proportion of water, with that which passed between the floats and the head, necessarily flows between them at the tail, the motion of the wheel is greatly impeded. On the contrary, by Mr. Besant's contrivance, no water can pass, except that which acts with all its force on the extremity of the wheel ; and, as the floats emerge from the water in an oblique direction, the weight of the atmosphere is thus prevent ed from taking any effect. A great ad vantarre of this construction is. that the wheel works are lighter, owing to a ten dency to floating. When working in deep tail-water it is decidedly superior, carry ing weight in the proportion of three to one Messrs. Polfreeman and Co. lately pur chased the patent-right given to Mr. Hawkins for his invention of floating water-mills, and established one, by per mission of the Board of Navigation, on the Thames. This kind of mill might be more generally used, with great advan tage to the public and, in lieu of being constructed as tide-mills (which require that the work should all revolve, either with or against the son), would, perhaps, be something improved by allowing the hulks, in which they are built, to swing round, like vessels at single anchor, in the tide's way.
Mitts, wind, are, in their general con struction, much the same as water-mills; adverting to the difference of the power by which they are acted upon. The ex ternal apparatus consists chiefly of the sails, or vanes, which are commonly fowl., placed in nearly a vertical position, and giving, as they turn, a rotatory motion to an axis inclining but little from the horizon. The form of the arms and vanes being so well known, we shall refrain from describing them in this The direction of the wind being ex tremely uncertain, it becomes necessary to have some contrivance for turning the sails towards it; for this purpose two modes are in more general use. In one, the whole machine is sustained upon a moveable arbor or axis, perpendicular to the horizon, and supported by a strong stand, or foot, very firmly fixed in the earth : thus, by means of a lever, the whole may be turned round to any direction. In the other way, only the upper part is moveable : the roof being a kind of cap, joining to the axis, on which the sails, or vanes, are fixed, and working round by means of an endless screw, that acts upon a ratchet frame, embracing the lower part of the cap. The former mode cpplies chiefly to what are called post-mills ; the latter, to built of masonry.
We offer the following table of veloci ty and power, resulting from the experi ments of Mr. Smeaton, and confirmed by Dr. Hutton, Mr. Rouse, &c. By it our readers will he able to compute to any extent, when on the subject of the wind's progress.
Whatever varieties may arise as to the internal structure of windmills, there are certain rules with regard to the position, shape, and magnitude of sails, which will bring them into the best state to receive the action of the wind, and to produce a full effect. M. Parent set his sails, or vanes, at an angle of 55° from the axis on which they project ; and that would have certainly proved the best, if no other ob ject than the acquisition of a certain de gree of velocity had been desirable ; but we find that from 72° to 75° gives a great er power , consequently, in their gene ral application, vanes standing at that an gle, or within one or two degrees more or less, are best calculated to produce a sufficient impetus for light breezes.
Mr. Smeaton made several experiments, which gave results proving the hypothe sis just stated. He had vanes set at the following angles, and found it better to give an excels of retirement from, than an excess of exposure to, the wind.
Angle with Ante with the Plano No. the elm of Motion.
1 . . . . 72° . ... 18° 2 . . . . 71 . . . . 19° S . . . . 72 . . . . 18° in the middle.
4 . . . . 74 . . . . 16° 5 . . . . 773 . . . . 12°4 6 . . . 83 . . . . 7° at the extremity.
He also tried the effects of a greater expanse of surface upon the same radius ; the result was, that a broader sail, in all cases, required a larger angle ; and that frustrated pyramidal sails, having their bases outwards, were more powerful than parallelograms, the extreme or outer bar being one-third the depth of the whip, or vane staff. Attempts were made to fill the whole space with sails; but it was evident, that, for want of sufficient pas sage for the wind, the intention was not fulfilled : when more than seves-eigliths of the area was spread with sail, there was an immense pressure, which caused much friction and imminent danger, while the velocity was rather diminished than augmented. Length of sail is a great ob ject, so far as relates to the acquisition of power, but where an excess prevails, ma ny injuries are sustained ; notwithstand ing the boom and guys, used in many places for the support of long arms.
As water-mills are, in general, stopped by shutting out the water, and thus de barring further influence of the power at pleasure ; so windmills are commonly stopped by a pinch, or pressure, on the axis bearing the vanes. Some are like wise acted upon by a weight which tends to retard the motion, and so slackens the rotation as to enable the pinch to have more effect. We have seen instances where the great axle could be cast off in an instant ; so that, although the vanes might continue to go round, the interior movements were stopped. This is an excellent contrivance, and may often save a mill from being burnt, when by acci dent, or neglect, the stones have come in contact, and produced collisive sparks. But in such case, it is obvious that a suffi cient counter-check should be created to retard the motion of the vanes; else they would, from want of due oppositioni move round with great rapidity, and produce other dangers no less imminent. This prevention is easily effected, by cauding the lever, which raises the main axle, to act against a stiff-set wheel, capable of checking its progress.
Some mills have a weathercock placed in the line with the axis, projecting se veral feet, and having sufficient surface to cause the cap to move round, so as al ways to keep the butt of the axle direct to the wind. This is an admirable expe dient, inasmuch as it effectually answers the intention, and supersedes the neces shy for the miller's constant attention to the wind ; which, when variable, occa sions considerable interruption to other avocations, and may, eventually, be at tended both with loss of time and some damage.
Mr. John Bywater, of Nottingham, ob tained a patent for clothing and uncloth ing the sails of windmills while in mo tion : his contrivance was nothing more than causing them to roll up lengthwise, by means of small wheels or ratchets, placed near the axis, and acted upon by it in its revolution.
Muds, horizontal wind, have likewise been tried, but they are both trouble some to manage and deficient in power : on the other hand, they are far safer, and cheaper in their construction than the vertical kind. The simplest mode of con structing a windmill is with a spiral sail, passing round a centre pole, tapering to wards the summit, and spreading to a great width at the base. This certainly has not very great powers, but acts with great uniformity, and requires no atten dance, since it matters not from what quarter the wind blows. We consider this machine to be very well calculated for raising water from fens, &c. both on account of its cheapness and its safety, even in the most exposed situations. The pole, or axis, to which the sail is fastened all the way up, being perpendicular, and every part presented to the wind giving it a tendency to rotation, while the main part being below, insures a steady action, and that absence of violent friction which cannot be effected in a vertical mill, or in any machine, where the greater part of the power is derived from the extremi ties of long arms or vanes. See Wirrn XILL.