“Certainly,” answered Mr. Seymour; “whenever a force continues to act, the motion produced by it must be accelerated for the reason already given you[17]--but let me explain the operation of the drop of water, which, as you have just seen, converted the sliding into the revolving motion. In the first place, in consequence of the cohesion of the water to the two surfaces, a new force was introduced, by which an unequal degree of resistance was imparted to different portions of that part of the watch-glass in contact with the plane, and, consequently, in its effort to slide down, it necessarily revolved. Now, if you will attentively observe the change of figure which the drop of water undergoes during the revolution of the glass, you will perceive a species of vortex; a film of water, by capillary action, is drawn to the foremost portion of the glass, while, by the centrifugal force, a body of water is thrown under the hinder part of it; the effect of both these actions is to accelerate the rotatory motion. “I shall now dismiss the subject for the present, but on some future occasion I shall probably revert to it; for it may be made to afford a simple illustration of the rotatory and progressive motions of the earth round the sun; and it may also give us the means of producing some optical effects of a very curious kind.”(15) Mrs. Seymour here suggested that, as it was past one o’clock, the children should be dismissed to their more active sports in the garden. “We will instantly proceed to the lawn,” replied Mr. Seymour, “and Tom may try his skill with the _sling_; an amusement which I have provided as a reward for his industry, and which will, at the same time, convey some farther information concerning the nature of those forces we have been just considering. The sling,” continued his father, as he advanced upon the lawn, “consists, as you perceive, of a leathern thong, broadest in the middle, and tapering off gradually towards both ends. To each extremity is affixed a piece of string. I shall now place a stone in the broad part of the leather, and introduce my middle finger into the loop formed in one of the strings, and hold the other extremity between my fore-finger and thumb.” He then whirled it round, and when it had gained sufficient impetus, he let go his hold of the string, and the stone instantly shot forth with amazing velocity. “See! see! there it goes,” exclaimed Tom; “to what a height it ascended!” “And to what a distance has it been projected!” observed Louisa, who had attentively watched its descent. “Now, Tom,” said his father, “can you explain the operation you have just witnessed?” “Not exactly, papa.” “Then attend to me. Have you not learned that circular motion is always the result of two forces?” “Undoubtedly,” replied Tom; “of one force which attracts it to the centre around which it moves, and of another which impels it to move off in a right line.”