“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.”