"You remember our long tube of mercury, of course," said Mr. Wilson. "I want you to think about it now. What was the height of the mercury in the tube?""The column of mercury is usually about 30 inches high, sir.""What causes it to rise in the tube?"
"The pressure of the air all round forces the mercury up the tube, sir.""And what is it that causes the water to rise in a pump?" "The water also rises in the pump because it is forcedup the pipe by the pressure of the air all round, sir." "Those are three very good answers," said Mr. Wilson.
"Now if we used the pump to raise mercury instead of water, how high do you think we could pump it? You must remember that the mercury, like the water, would rise only by reason of the pressure of the air.""I suppose, sir," said Fred, "it would be impossible to raise the mercury beyond 30 inches.""You are quite right, Fred. You know that a column of mercury 30 inches high, and a square inch in section, weighs just 15 lbs., and this exactly balances the pressure of the atmosphere on the square inch.
"We might have used water in the tube instead of mercury, but as water is 13? times as light as mercury, our tube must have been 13? times as long, because we should have required 13? times as much water to balance the weight of the air. Our tube must then have been 30 inches × 13? = 34 feet (nearly).
"Now, of course, you see clearly why we use mercury instead of water. It requires 34 feet of water to balance 30 inches of mercury-that is, to balance the pressure of the air.
"I wonder whether you can tell me now how high we can raise water by means of a pump?""I should think the water would rise about 34 feet, sir," said Fred.
"Well, it ought to rise to that height, Fred, but it does not, because we cannot get a perfect vacuum, unless the pump is constructed with the utmost care. The greatest height we can raise water with a common pump is from 26 to 28 feet.
"Now although we cannot raise water higher than this with an ordinary pump, it is often found necessary to carry water to many times this height. Let me explain how it is done.
"For this purpose a pump, called the Lifting Pump, is used. The main difference between it and the common pump is that the piston of this one has no valve. Near the bottom of the barrel is a discharge-pipe which bendsupwards, and may be carried to any height. At the entrance to this pipe is a valve, opening outwards. It allows the water to pass out of the barrel, but prevents it from returning.
"The working is very simple. As soon as the water reaches the barrel in the ordinary way, the next downward move of the solid piston must force it somewhere. The water itself, you know, will not be squeezed into smaller bulk. It cannot go back into the suction-pipe, because itsvalve is closed firmly. The other valve, however, leadinginto the discharge-pipe is open, and, at the first pressure of the piston, the water takes that course. A pump of this kind will lift water to almost any height."Lesson 55
