英文科學讀本 第四冊·Lesson 42 The Tube of Mercury

Lesson 42 The Tube of Mercury

“You remember our long glass tube of mercury, of course,” said Mr. Wilson. “The column of mercury filled the tube to the height of about 30 inches, and no shaking of the tube could move the liquid so long as we kept the lower end in the bowl of mercury. I think you are now in a position to explain why this quantity of the metal remains in the tube.”

“The column of mercury 30 inches high weighs exactly the same as the long column of air,” said Fred, “the two balance each other.”

“You are quite right, Fred. We will now take the next step. I will draw a little square on this slate with the help of the ruler. Each side of our square shall measure exactly 1 inch, and we may call it a square inch. We will place the slate on the table, and I want you to imagine a tube 30 inches high, built up on this little square, as its base. Imagine, further, this tube filled with mercury. The quantity of mercury required to fill it would weigh just 15 lbs.

“Now if our tube in the last lesson had been as big as this one (a square inch in section), our column of air pressing down on the surface of the mercury outside must also have been a square inch—that is to say, it must have covered a square inch of mercury in the basin.

“What do we learn, then, from this? It is clear that the pressure of the air on a square inch of surface would be balanced by a column of mercury weighing 15 lbs. In other words, the air presses downwards with a force of 15 lbs. on every square inch of surface.

“Now let us go a little further,” he continued. “We will begin by measuring this slate. It is just 10 inches long and 8 inches across. I will mark the inches on the sides and draw lines across the slate from mark to mark. You see I have marked out the slate into a great number of squares, each of them measuring an inch every way. They are square inches. Count them, Fred, and see how many square inches there are altogether.”

“There are 80 square inches, sir.”

“Very well; then the surface of the slate must bear a pressure of 80 times 15 lbs., or upwards of half a ton. But a baby could lift the slate. It is not heavy. Why is this? There is not only that enormous pressure on the top of the slate; there is an equal pressure from below and all around.

“Think what must be the pressure on this table. But with it all the table is not crushed, because there is an upward pressure of equal force to balance it, and it is not felt.

“Every child goes about with many tons pressing upon his body, but he never feels it, because there is air inside his body too, and that balances the pressure on all sides of him.

“This will help us to understand better why our leather sucker holds fast. Suppose the sucker measured 6 square inches; there would then be a pressure of six times 15 lbs., or 90 lbs. pressing it to the slate. We felt that pressure when we pulled the string, because there was no upward pressure between the slate and the leather to balance it. The pressure of 90 lbs. on the top pressed the sucker and the slate firmly together, and we could not move them.

“When the hole was bored in the leather, and air rushed in, we had the same force of 15 lbs. on every square inch of the under surface, and the sucker let go the stone; the two pressures balanced each other.”