英文科學讀本 第四冊·Lesson 45 The Barometer

Lesson 45 The Barometer

“I want you to think about our long tube of mercury again,” said Mr. Wilson. “Here it is. The mercury stands in the tube, and you know it balances the air which is pressing on the surface of the mercury in the basin. It stands now at about 30 inches, but what would happen if the pressure of the air were to change?”

“The quantity of mercury in the tube would change too, I suppose, sir,” said Fred.

“Quite right, Fred. If the air pressed with greater pressure, more mercury would be forced up the tube. If the air pressure were less than it now, some of the mercury would flow out into the basin; it would not stand so high.

“Let us in imagination take our mercury tube up with us in a balloon. If we look at it after we have ascended 1000 feet, we shall find that the mercury has fallen 1 inch, and it would continue to fall at the same rate for every 1000 feet of our ascent. On the top of Mont Blanc, 15,000 feet high, the column would measure only 15 inches instead of 30 inches, and would weigh only 7 1/2 lbs. instead of 15 lbs. You know that the air gets thinner and thinner the higher we ascend, and the mercury proves that it also gets lighter, and of course presses with less force.

“On the other hand, if we took our mercury tube down into a coal mine we should find that the lower we went the higher the mercury would rise. It would rise, roughly speaking, at the rate of 1 inch for every 1000 feet we descended. The weight and pressure of the air in the mine increases with the depth.

“Now I want you to think of something else. You know that the air always contains more or less water-vapor, and that this vapor rises in the air. It is very clear then that it must be lighter than the air. Air which contains much moisture is always lighter than dry air. Dry air is heavy; moist, damp air is light. Now how would the condition of the air affect our mercury tube? When the air is dry and therefore heavy, it presses with great force on the mercury in the basin, and forces more mercury up into the tube; the column of mercury is high. When the air is full of vapor and therefore light, the pressure is less, and the column of mercury in the tube falls.

“You can see from this that the mercury in the tube measures exactly the pressure or weight of the atmosphere, and I am now in a position to tell you that we have all this time been studying a very useful and wonderful instrument. That which I have purposely spoken of as a mercury tube has a proper name. We call it barometer. The word means ‘the weight measurer.’ It measures the weight or pressure of the air.

“Our tube and basin of mercury would be very awkward to carry about from place to place. This is why, for the sake of safety, the tube is fitted into a frame, with a little cistern at the bottom instead of the basin. The height of the column is marked in inches at the side of the tube, and the instrument is complete.

“The barometer is used for measuring the heights of mountains, and you will readily see the reason why.

“It is also used as a weather-glass. If the air contains a large amount of water-vapor, we know that we are likely to get rain and stormy weather. If the air is dry, there is a prospect of fine weather. The barometer, in either case, gives us timely warning of the weather to be expected, by registering the pressure of the air.”