On a pedestal is placed a small tree round which a serpent or dragon is coiled; a figure of Hercules stands near shooting from a bow, and an apple lies upon the pedestal: if any one raises, with the hand, the apple a little from the pedestal, Hercules shall discharge his arrow at the serpent and the serpent hiss. Let A B (fig. 40) be the proposed pedestal, air-tight and divided by a partition, C D. Fixed in the partition is a hollow truncated cone, E F, the lesser circle of which, F, is open and approaches to the bottom of the pedestal, leaving a sufficient interval for the passage of water. To this cone must be tightly fitted another cone H, attached by means of a chain through a hole in the surface, to the apple K, which lies on the pedestal. Let Hercules hold a small bow of horn, the string of which is stretched, and at the proper distance from the hand. In the right hand, and directed towards the serpent, let there be a hand in every respect similar to the visible hand, but smaller, and holding the trigger. From the extremity of the trigger let a chain, or cord, proceed through the pedestal and be attached to a pulley, which is placed above the partition, and again to the chain which is connected with the cone and apple. Now we must draw the bow, and placing the trigger beneath the hand, close it so that the cord is stretched and draws the apple tightly downwards: the cord must run inside Hercules and through the body and hand. From the partition let a small tube, one of those which are used to whistle, extend above the pedestal and pass under the tree or along its trunk. Then fill the vessel A D with water. Let L M be the tree, N X the bow, S P the string, R S the hand that grasps the bow, T U the trigger, Q W the cord, W the pulley round which the cord runs, and Y Z the whistling pipe. Now if some one raise the apple K, he will at the same time raise the cone H, tighten the cord Q W, and draw back the hand, so that the arrow is discharged: and the water in A D, being carried into B C, will drive out the air contained in B C through the pipe, and produce the hissing sound. When the apple is replaced, the cone H fitting again into the other, will stop the stream of water so that no sound is produced. We must now re-arrange the arrow and leave it. If the vessel B C is full, it can be emptied again by means of a spout with a key: A D must be filled as before. 41. _A Vessel from which uniform Quantities only of Liquid can be poured._ [Illustration: Diagram of apparatus as described in text]