When it withdraws itself, the Stopper closes the mouth of the tube with perfect exactness, so as to leave the inhabitant in safety. The reader will see, on referring to the illustration, how exactly similar is the Conical Stopper of Art to that of Nature, and how the inventor of that article, as well as of the self-fitting candle, the candle-fixer, the blow-gun arrow, and the vent-peg, might have found prototypes of their inventions in Nature, if they had only known where to look for them. THE FILTER. Even in a state of uncivilisation man has been driven to invent a Filter of some kind. The simplest kind of Filter is that which is used by the Bosjesman women when procuring water for the use of their families. When, as often happens, the only water to be obtained is to be found in muddy pools which have been trampled and perturbed by thirsty animals, the women have recourse to a simple, though rather repulsive, expedient. Each woman is furnished with empty ostrich egg-shells by way of water-vessels, and she also takes a couple of hollow reeds. Over the end of one of these reeds she ties a bundle of grass, and then plunges it as deeply as she can into the mud. After a little while she sucks up the water through the tube, the grass acting as a filter, and she then discharges it by the second tube into the egg-shells. In this way the women will obtain water, where none but themselves could have procured it. As to the repulsive mode of obtaining it, no one can be fastidious when dying of thirst. Sir S. Baker mentions that when he was on his travels he managed in a halt to save up enough water for a bath for himself and his wife. He was about to throw away the soapy water, when the vessel was snatched from his hands by two of his attendants, and the contents eagerly drunk. The different varieties of the Filter which we use at the present day are too familiar to need description. Whether they be made principally of charcoal, which is a powerful disinfectant, or of merely stones, gravel, and sand, they are all constructed on the same principle, namely, the straining out solid substances, and allowing only the pure water to pass through the interstices. * * * * * AS to the Filters of Nature, they are almost innumerable. In the first place, the Earth itself is the primary filter of all, taking into itself all kinds of decomposing substances, separating them for the use of vegetation, and delivering the pure, bright, and sparkling spring water which we so highly and rightly value. The whole human body, again, is practically a collection of the most elaborate and effective filters that the mind of man can conceive. But we will pass to the more obvious examples of filters as seen in animal life.