1218

THEORY AND PRACTICE OF ENGINEERING.

Book II.

is a barometrical gauge or tube 6 or 8 inches in length, sealed at one end and filled withmercury ; as soon as the tension of the contained air becomes less than sufficient to supportthe column of mercury, the liquid begins to fall, and the height at which it stands abovethe level of that, on the basin into which It is plunged, is the measure of the tension of theremaining air.

On the subject of the u-inds much has been written, but the various theories are involved inconsiderable obscurity ; its velocity varies from zero up to 100 miles an hour.

Po*t Windmills are' much in use in many parts of England for grinding corn or cement,but their size being limited, they are not generally applied to drive machinery for otherpurposes. The mill is usu-ally made on a rectangularplan, and the narrow sideturned to the wind. Thewind-shaft bearing on abeam at each end lias a littleinclination, and on the out-side is increased in di-mension, forming a squarefor the better mortising thewhips or arms of the sails,which are wedged tightlyinto it. The inclined wheelattached to the wind-shafthas on its circumference arim of wood, called thebrake, one end of which isfixed to the side of the mill,and the other, by an iron rod,to a lever, which can bepressed down, and made tobind upon the circumferenceof the wheel, with sufficientpower to stop the mill.

The whole mill revolvesround the upright post inthe centre, which is framedat bottom into some longhorizontal timbers secured tothe ground, from which fouroblique braces are strutted,to maintain it in its verticalposition. "Where this up-

Fig. 1914.

POST MII-L.

right post passes, the floor of the mill is a circular iron collar, and at the upper end of thepost is a. pivot or gudgeon, entering a socket, let into one of the timbers, made of con-siderable strength, as the entire weight of the mill rests upon it. The mill thus balancedis turned by the ladder which serves as a lever ; after the right position is obtained, it iskept steady by dropping the ladder on the ground ; this is readily performed by a rope andpulley, or by manual labour.

The Tower or Smock Mill was formerly more common, but since the general introductionof the steam-engine it has gone out of use in many districts. The body of the mill isconstructed either in brick or stone, and sometimes of timber ; at the top of the building awooden curb is laid, or strong wall plate, on which the rollers of the cap revolve, and it isessentially necessary that this portion of the work should be rendered very secure, or therattle and motion of the machinery will loosen and derange it, or cause great inconvenience.

In the outer end of the wind-shaft, which is commonly made of iron, are cast two socketsfor the insertion of the whips of the sails, which are tightened in their place by woodenwedges. The pillows which bear the wind-shaft are of brass, and sometimes of hard stone.The driving or brake wheel is bevelled, and furnished with two rows of teeth, which actalternately upon those on the bevelled crown wheel, on the top of the upright shaft, and bythis arrangement a more regular motion is obtained : these hit and miss wheels, commonin many parts of England, are admirably adapted to windmills.

The brake wheel has a wooden rim, one end of which is attached to the framing of themill and the other to a lever: a rope passing over a pulley to the outside of the millenables the miller to raise the end of the lever, and engage it with a hook, which liberatesthe brake, and allows the head of the mill to move freely round : when it is required tostop the mill by pulling the rope, the end of the lever is disengaged from the hook, whichfalling on the brake brings it down upon the wheel, and stops all motion even during the