“That is intelligible enough,” said Tom, “the feather of the peacock must have been so flexible as to have yielded to the slightest breath of air; and now, as we are upon the subject of the arrow, do explain to us the action of the bow.”
“I shall readily comply with your request, before we part; but I am desirous, at present, of following up the subject before us, and of taking into consideration some other instruments which owe their motions to the action of the air upon oblique surfaces.”
“Suppose,” said the vicar, “you explain to them the action of the wind upon the sails of the mill.”
“I should like to hear something about the windmill,” observed Tom; “and, perhaps, Mr. Twaddleton can tell us who invented the machine.”
“The invention is not of very remote date,” replied the vicar. “According to some authors, windmills were first used in France in the sixth century; while others maintain that they were brought to Europe in the time of the crusades, and that they had long been employed in the East, where the scarcity of water precluded the application of that powerful agent to machinery.”
“I had intended,” said Mr. Seymour, “to have entered very fully upon the subject of the windmill; for, although it is a very common machine, its construction is much more ingenious than is generally imagined; it must also be allowed to have a degree of perfection, to which few of the popular engines have yet arrived: but to do ample justice to my subject, I should require several models which are not yet in readiness; besides, Tom’s holidays have nearly passed away; I must therefore postpone the examination of the mill to some future opportunity, and content myself, at present, with an explanation of its sails.”
“And let me tell you,” observed the vicar, “that if you succeed in this one object, you will accomplish a task which has occupied years of mechanical research. The angle which the surface of the sails ought to make with their axis, in order that the wind may have the greatest effect, or the degree of weathering, as the millwrights call it, is a matter of nice enquiry, and has much engaged the thoughts of the mathematicians.”
“My remarks upon that subject will be very general,” said Mr. Seymour; “I shall explain the principle, without entering into the minutiæ of its applications. The vertical windmill, which is the kind in most common use, consists, as you well know, of an axis, or shaft, placed in the direction of the wind, and usually inclining a little upwards from the horizontal line. At one end of this, four long arms, or yards, are fixed perpendicular to the axis, and across each other at right angles; these afford a surface, on which a cloth can be spread to receive the action of the wind. To conceive why these sails should revolve by the force of the wind, we must have recourse to the theory of compound motion. It is very evident that, if a mill exposed directly to the wind should have its four sails perpendicular to the common axis in which they are fitted, they would receive the wind perpendicularly, an impulse which could only tend to overturn them; there is a necessity, therefore, to have them oblique to the common axis, that they may receive the wind obliquely, when their effort to recede from it causes them to tarn round with the axis; and the four sails being all made oblique in the same direction, thus unite their efforts for the common object.”
“You have not yet told us what degree of obliquity the sail ought to make with the wind,” said the vicar.
“The same as the kite ought to make, fifty-four degrees and forty-four minutes,” replied Mr. Seymour.