15. Birds made to sing, and be silent alternately by flowing
The construction is after this manner. Let A (fig. 15) be a stream perpetually running. Underneath place an air-tight vessel, B C D E, provided with an inclosed diabetes or bent siphon F G, and having inserted in it a funnel, H K, between the extremity of the tube of which and the bottom of the vessel a passage is left for the water. Let the funnel be provided with several smaller pipes, as described before, at L. It will be found that, while B C D E is being filled with water, the air that is driven out will produce the notes of birds; and as the water is being drawn off N through the Siphon F G after the vessel is filled, M the birds will be mute. We are now to describe the contrivance by which the owl is enabled to turn herself towards, or away from, the birds as we have said. Let a rod N X turned in a lathe rest on any support M: round this rod let a tube O P be fitted, so as to move freely about it, and having attached to it the kettle-drum top R S, on which the owl is to be securely fixed. Round the tube 0 P let a chain pass, the two extremities of which, T U, Q W, wind off in opposite directions, and are attached, by means of two pullies, the one, T U, to a weight suspended at Y, and Q W to an empty vessel Z, which lies beneath the siphon or inclosed diabetes F G. It will be found that while the vessel B C D E is being emptied, the liquid being carried into the vessel Z causes the tube 0 P to revolve, and the owl with it, so as to face the birds: but when B C D E is exhausted, the vessel Z becomes empty likewise by means of an inclosed or bent siphon contained within it; and then the weight Y, again preponderating, causes the owl to turn away just at the time when the vessel B C D E is being filled again and the notes once more issue from the birds.