A mill of any kind actuated by the impulse of the wind. They are of two kinds - vertical and horizontal.

Vertical windmills (to which a decided preference has been hitherto given) usually consist of a strong shaft or axis inclining a little upwards from the horizon, with four long yards or arms fixed to the highest end, perpendicular to the shaft, and crossing each other at right angles. Into these arms are mortised several small cross bars, and to them are fastened two, three, or four long bars, running in a direction parallel with the length of the arms; so that the bars intersect each other, and form a kind of lattice-work, on which the cloth is spread to receive the action of the wind. These are called the sails, and are in the shape of a trepezium, usually about nine yards long and two wide. The direction of the wind being always very uncertain and variable, it becomes necessary to provide some contrivance for bringing the sails into a proper position for receiving its impression. Two methods have been devised for this purpose, one of which is denominated the post-mill, the other, the smock-mill. The post-mill is so called from the circumstance of the mill being built round a massive central post, made out of the whole trunk of a stout tree, which is sunk vertically in the ground, and supported in its position by oblique struts or braces, which extend from a platform on the ground to the middle of the post, leaving 10 or 12 feet of the upper part free from the braces.

The part thus left free from obstruction is rounded and made to pass through a circular collar, formed in the flooring of the lower chamber, and to enter into a socket fixed into the flooring of the upper chamber, and to one of the strongest cross-beams, which must sustain the whole weight of the mill-house; so that by means of a pivot, or gudgeon, fastened on that part of the post which enters into the socket, the whole machine can turn about horizontally to face the wind. A strong framing, united by joints at the back of the mill-house, descends in a sloping direction to the ground, and is there fastened to short posts, when placed in the position required for the sails to be acted upon by the wind. To this frame a adder is attached, which leads into or out of the mill-house. To the bottom of this frame a rope is fastened and conducted to tackle in the mill-house, by which the frame can be lifted from the ground, while its position is being changed, in the manner of a capstan post, to suit the wind.

The smock-mill does not depend upon a central post for its main support, but it is generally a strong independent building, the upper portion of which is usually a tower of the form of a truncated cone, constructed of wood, and mounted upon a vertical wall of masonry, containing two or three floors, where the work of the mill is performed, - the tower above containing a vertical shaft, by which the motion and force is communicated from the sails to the mill-stones. The head or cap in the upper part of the mill is provided with a cap, which is contrived so that it may turn itself about as the wind changes; for this purpose there is a nearly horizontal framed projection at the back part of the head, which carries some small sails acting as a vane, there being, concentric with the axis, a large grooved ring, around which a circular hoop, provided with anti-friction rollers, traverses.

The velocity of motion of the sails or vanes is very considerable. Mr. Ferguson calculated the motion of the tips of the sails, even when operated upon by a very moderate wind, to be thirty miles per hour.

Horizontal windmills, as their name implies, are such as are worked by their sails revolving in a horizontal plane. All disinterested authors who have written on this subject condemn them, as being very inferior in effect to those of the vertical kind. Smeaton considered their effect to be only one-eighth, but Dr. Brewster shows that they have from one-third to one-fourth of the effect of the vertical. It is probable, however, that means may be discovered of improving them considerably.

To ascertain the best form and position of windmill-sails, Mr. Smeaton instituted a series of experiments, of which the results are given in the subjoined table: -

Table, Exhibiting The Results Of Nineteen Sets Of Experiments On Windmill Sails, Of Various Structures, Positions, And Extents Of Surface

THE DESCRIPTION OF SAILS MADE USE OF.

Number.

Angle at the Extremities,

Greatest Angle.

Turns of the Sails unloaded.

Turns of the Sails at the Maximum.

Load at the

Maximum.

Greatest Load.

Product.

Extent of Surface.

Ratio of greatest Velocity to the Velocity at a Maximum.

Ratio of greatest Load to the Load at a Maximum.

Ratio of

Surface to the

Product.

Degrees.

Degrees.

lb.

lb.

Sq. In.

Plane Sails, at an angle of 55°

1

35

35

66

42

7.56

12.59

318

404

10:7

10:6

10:7.9

Plane Sails, weathered ac-cording to the common practice . . . . . . . . . . . . . . .

2

12

12

___

70

6.3

7.56

441

404

.

10:8.3

10:10.1

3

15

15

105

69

6.72

8.12

464

404

10:6.6

19:8.3

10:10.15

4

18

18

96

66

7.0

9.81

462

404

10:7

10:7.1

10:10.15

Weathered according to M'Laurin's Theory . . . . . . .

5

9

26 1/2

-

66

7.0

-

462

404

__

__

10:11.4

6

12

29 1/2

-

70 1/2

7.35

-

518

404

-

-

10:12.8

7

15

32 1/2

-

63 1/2

8.3

-

527

404

-

-

10:13.

Sails weathered in the Dutch manner, tried in various positions . . . . . . . . . . . . . .

8

0

15

120

93

4.75

5.31

442

404

10:7.7

10:8.9

10:11.

9

3

18

120

79

7.0

8.12

553

404

10:6.6

10:8.6

10:13.7

10

5

20

___

78

7.5

8.12

585

404

--

10:9.2

10:14.5

11

7 1/2

291/2

113

77

8.3

9.81

639

404

10:6.8

10:8.5

10:15.8

12

10

25

108

73

8.69

10.37

634

404

10:6.8

10:8.4

10:15.7

13

12

27

100

66

8.41

10.94

580

404

10:6.6

10:7.7

10:14.4

Sails weathered in the Dutch manner, hut enlarged to-ward3 the Extremities. . .

14

7 1/2

22 1/2

123

75

10.65

12.59

799

505

10:6.1

10:8.5

10:15.8

15

10

25

117

74

11.08

13.69

820

505

10:6.3

10:8.1

10:16.2

16

12

27

114

66

12.09

14.23

799

505

10:5.8

10:8.4

10:15.8

17

15

30

96

63

12.09

14.78

762

505

10:6.6

10:8.2

10:15.1

Eight Sails, being Sectors of Ellipses, in their best positions . . . . . . . . . . . . .

18

12

22

105

64 1/2

16.42

27.87

1059

854

10:6.1

10:5.9

10:12.4

19

12

22

99

64 1/2

18.06

-

1165

1146

10:5.9

-

10.10.1