“It makes it adhere to it by some kind of suction, but I confess that I do not exactly understand the subject.”
“Then let us proceed cautiously and deliberately in the explanation. In the first place, you have said, and said correctly, that the edges of the leather adhere to the stone; but what is the nature of the power to which this adhesion is to be referred? I perceive you are puzzled by the question: attend, then, to my explanation: you must know that there exists a tendency in all bodies to adhere together, provided the contact of their surfaces be sufficiently perfect; this property is termed cohesion, or cohesive attraction, from the Latin word cohæreo, which I need not inform you signifies to stick together. The dry leather will not adhere to a smooth surface, because, in that case, the contact cannot be rendered sufficiently perfect; but, when saturated with water, the interstices of the leather are filled with that fluid, and the inequalities of the surface, which must always prevent close contact, are removed. If two bodies, when placed together, be not sufficiently smooth, or polished, it will be vain to make any attempt to produce their cohesion; since the particles will, in such a state, touch each other only in a few points; it is for this reason that carpenters, when they intend to glue pieces of wood together, plane the surfaces perfectly smooth, before they apply the glue.”
Tom here acknowledged that he had not before understood the reason of the leather’s adhesion to the stone.
“Having, then, settled this point to your satisfaction,” continued Mr. Seymour, “let us proceed. Your idea of a vacuum being formed in the hollow part of the leather is perfectly correct: for, as you draw up the central part by the string, the hollow thus produced must necessarily be a vacuum, since the air cannot pass through the leather to supply it; in this state, therefore, the atmosphere presses upon the exterior of the leather, and like any other weight prevents its rising from the stone.”
Fanny and Louisa here expressed some surprise on hearing of the weight of the atmosphere; the former observed, that she did not feel any pressure from it. Their father explained the reason of their not being conscious of the weight, by informing them that their bodies contained air, which, by its elasticity, counteracted the pressure from without; but that, if it were possible to remove all the air which the body contained, the pressure of the atmosphere would not be counteracted; and the consequence would be, that we should be flattened like a pancake by its weight, which had been ascertained by experiment to be equal to fifteen pounds upon every square inch of surface, or, as much as forty thousand pounds upon the body of a man of ordinary size.
“Until your explanation,” said Tom, “I really believed that the leather adhered to the stone by some kind of suction, just as the back of my hand adheres to my lips, whenever I place it to my mouth, and draw in my breath.”
Mr. Seymour here expressed a doubt whether his son was even yet a perfect master of the subject: he told him that there was no such operation in nature as suction; that it was merely a popular term to denote the action of the air upon a vacuum. “Your hand,” said he, “adheres to your mouth, in consequence of your forming a vacuum within it, by forcibly drawing in your breath, and the resistance which is opposed to its removal arises entirely from the pressure of the atmosphere upon it. Many are the effects which may be explained upon a similar principle. I dare say you well remember the astonishment which you expressed at the force with which the limpets attached themselves to the rocks.”
“O yes, papa,” exclaimed Louisa, “I well remember, when we walked on the sea-shore, that, on first touching the limpets, they appeared loose and moveable, but before I had time to remove them, they fastened themselves as firmly as though they had been a part of the rock upon which they were fixed; how could that happen?”
Mr. Seymour replied, that these sea-insects possessed the power of converting their whole bodies into suckers; and he informed them, that many other animals were endowed with a similar faculty. He instanced the claws of the polypus, which are furnished with many such suckers, by means of which the animal is enabled to hold to whatever it attaches itself, with very considerable force.
“Have you never observed,” asked Mr. Seymour, “the security and ease with which flies frequently walk upon a smooth wall, or a pane of glass, or even along the ceiling, with their bodies downward?”