Now when we wish to see the weight of the air we consult a barometer, which works really just in the same way as the water in this tube. An ordinary upright barometer is simply a straight tube of glass filled with mercury or quicksilver, and turned upside-down in a small cup of mercury. The tube is a little more than 30 inches long, and though it is quite full of mercury before it is turned up, yet directly it stands in the cup the mercury falls, till there is a height of about 30 inches between the surface of the mercury in the cup, and that of the mercury in the tube. As it falls it leaves an empty space above the mercury which is called a vacuum, because it has no air in it. Now, the mercury is under the same conditions as the water was in the U tube, there is no pressure upon it at the top of the tube, while there is a pressure of 15 lbs. upon it in the bowl, and therefore it remains held up in the tube.
But why will it not remain more than 30 inches high in the tube? You must remember it is only kept up in the tube at all by the air which presses on the mercury in the cup. And that column of mercury now balances the pressure of the air outside, and presses down on the mercury in the cup at its mouth just as much as the air does on the rest. So this cup and tube act exactly like a pair of scales. The air outside is the thing to be weighed at one end as it presses on the mercury, the column answers to the leaden weight at the other end which tells you how heavy the air is. Now if the bore of this tube is made an inch square, then the 30 inches of mercury in it weigh exactly 15 lbs, and so we know that the weight of the air is 15 lbs. upon every square inch, but if the bore of the tube is only half a square inch, and therefore the 30 inches of mercury only weigh 7 1/2 lbs. instead of 15 lbs., the pressure of the atmosphere will also be halved, because it will only act upon half a square inch of surface, and for this reason it will make no difference to the height of the mercury whether the tube be broad or narrow.
But now suppose the atmosphere grows lighter, as it does when it has much damp in it. The barometer will show this at once, because there will be less weight on the mercury in the cup, therefore it will not keep the mercury pushed so high up in the tube. In other words, the mercury in the tube will fall.
Let us suppose that one day the air is so much lighter that it presses down only with a weight of 14 1/2 lbs. to the square inch instead of 15 lbs. Then the mercury would fall to 29 inches, because each inch is equal to the weight of half a pound. Now, when the air is damp and very full of water-vapour it is much lighter, and so when the barometer falls we expect rain. Sometimes, however, other causes make the air light, and then, although the barometer is low, no rain comes,
Again, if the air becomes heavier the mercury is pushed up above 30 to 31 inches, and in this way we are able to weigh the invisible air-ocean all over the world, and tell when it grows lighter or heavier. This then, is the secret of the barometer. We cannot speak of the thermometer today, but I should like to warn you in passing that it has nothing to do with the weight of the air, but only with heat, and acts in quite a different way.