The boy’s well-known toy, the Sucker, is made on exactly the same principle. A piece of leather, generally circular, though the shape is not of much consequence, has a hole bored through its centre, so as to allow a string to be attached. The leather is then soaked in water until it is quite soft. If it be firmly pressed on any smooth object, such as a stone, the air is forced from under it, and it becomes capable of sustaining a weight in proportion to its dimensions. As the air has a pressure of about fifteen pounds on every square inch, it is easy to calculate the weight which it will uphold, a margin being left for imperfection of vacuum. [Illustration: SUCKERS OF CUTTLE. LIMPET. STAR-FISH. SUCKING-FISH. FOOT OF GECKO. LAMPERN. CUPPING-GLASS. PNEUMATIC PEG. SUCKER. MAGDEBURG HEMISPHERES.] * * * * * THE lower figure represents the instrument called the Magdeburg Hemispheres, which are made for the purpose of showing the enormous power of air-pressure. They are two hollowed hemispheres, having their edges very accurately ground together. When used, a little lard is rubbed on the edges in order to insure their exact fit, and they are then pressed tightly together. The air is removed by means of the common exhausting syringe, and it is found that the two adhere together with such force that two strong men cannot pull them asunder. But, if the tap be turned, and air admitted, they come apart without the least difficulty. Similarly, if two plates of glass or metal be ground to exactly plane surfaces, and pressed together, they adhere nearly as strongly as if they were one solid piece. * * * * * WE will now turn from Art to Nature, and examine some natural producers of vacuum. One of the most celebrated is that series of suckers which may be found upon the arms of the various Cuttles. At the upper part of the illustration a figure is given of part of an arm, on which are four suckers. When the animal wishes to attach itself to any object, it presses the disc of the sucker against it, and simultaneously withdraws the centre, exactly as the boy does with his toy sucker. And, as each arm contains a great number of suckers, it is evident that the holding power must be very great. Indeed, on one occasion when a comparatively small specimen had fastened on a man’s arm, he could not remove it, but was obliged to have it cut away piecemeal by an assistant. The common Water-beetle has similar suckers upon its first pair of feet, and can adhere to smooth surfaces with great tenacity. * * * * *