_Compound Forces.--The Composition and Resolution of Motion.--Rotatory Motion.--The Revolving Watch-glass.--The Sling.--The Centrifugal and Centripetal Forces.--Theory of Projectiles.--A Geological conversation between Mr. Seymour and the Vicar._ On the following morning, Mr. Seymour proceeded to explain the nature of “COMPOUND FORCES.” The young party having assembled as usual, their father commenced his lecture by reminding his auditors, that the motion of a body actuated by a single force was always in a right line, and in the direction in which it received the impulse. “Do you mean to say, papa, that a single force can never make a body move round, or in a crooked direction; if so, how is it that my ball, or marble, will frequently run along the ground in a curved direction? Indeed, I always find it very difficult to make it go straight.” “Depend upon it, my dear, whenever the direction of a moving body deviates from a straight line, it has been influenced by some second force.” “Then I suppose that, whenever my marble runs in a curved line, there must be some second force to make it do so.” “Undoubtedly; the inequality of the ground may give it a new direction; which, when combined with the original force which it received from your hand, will fully explain the irregularity of its course. It is to the consideration of such compound motion that I am now desirous of directing your attention: the subject is termed the “COMPOSITION OF FORCES.” Here is a block of wood, with two strings, as you may perceive, affixed to it: do you take hold of one of these strings, Louisa; and you, Tom, of the other. That is right. Now place the block at one of the corners or _angles_ of the table: and while Tom draws it along one of its sides, do you, Louisa, at the same time, draw it along the other.” The children obeyed their father’s directions. [Illustration: A piece of wood with two strings attached to it, and two hands pulling the strings in diagonal directions.] “See!” said Mr. Seymour, “the block obeys neither of the strings, but picks out for itself a path which is intermediate. Can you tell me, Tom, the exact direction which it takes?” “If we consider this table as a parallelogram, I should say, that the block described the diagonal.”