"Then, what is it that is so heavy?"

"Come, now! Try once more, with all your strength."

Miette endeavored to lower the right-hand side of the lever: she could not succeed.

"Why," said she, "it is, of course, the piston on the left which has become too heavy, as I cannot make it rise again."

"You are right, Miss Miette. It is the piston in the left cylinder which cannot rise; but it has not changed its weight, as I said,--only it has now to support a very considerable weight; and it is that weight which you cannot combat."

"What weight is it?" said Miette, who did not understand.

"The weight of the air."

"The weight of the air? But what air?"

"The air which is above it,--the exterior air; the air which weighs down this piston, as it weighs us down."

"Does air weigh much?"

"If you are very anxious to know, I will tell you that a wine gallon of air weighs about seventy-two grains; and as in the atmosphere--that is to say, in the ma.s.s of air which surrounds us--there is a very great number of gallons, you can imagine that it must represent a respectable number of pounds. It has been calculated, in fact, that each square inch of the surface of the soil supports a weight of air of a little more than sixteen pounds."

"But how is that?" cried Miette. "A while ago there was also a considerable quant.i.ty of air above the piston, and yet I could make it go up very easily."

"Certainly, there was above the piston the same quant.i.ty of air as now, but there was air also in the globe. Air, like gas, possesses an elastic force,--that is to say, that it constantly endeavors to distend its molecules, and presses without ceasing upon the sides of the vase which contained it, or upon the surrounding air. Now, when you began to work the lever there was still enough air in the globe to balance, through its elastic force, the air outside; and, as the piston receives an almost equal pressure of air from the atmosphere above and from the globe below, it is easily raised and lowered. But while you were working the lever you took air out of the globe, so that at last there arrived a time when so little air remained in this globe that its elastic force acted with little power upon the piston. So the piston was submitted to only one pressure,--that of the atmosphere; and, as I have just told you, the atmosphere weighs heavy enough to withstand your little strength. Still, all the air in the globe is not yet exhausted, and a stronger person, like Master Paul, for example, could still be able to conquer the resistance of the atmosphere and raise the piston."

Paul Solange could not refuse this direct invitation, and he approached the air-pump and succeeded in working the lever, though with a certain difficulty.

Meanwhile, Monsieur Roger was seeking among the physical instruments which had just arrived. He soon found a gla.s.s cylinder, whose upper opening was closed by a bit of bladder stretched taut and carefully tied upon the edges.

"Stop, Master Paul," said he: "we are going to exchange the globe for this cylinder, and you will see very readily that the air is heavy. Now take away the globe."

But, though Paul tried his best, he could not succeed in obeying this order. The globe remained firm in its place.

"That is still another proof of the weight of the air," said Monsieur Roger. "The globe is empty of air; and as there is no longer any pressure upon it except from outside,--the pressure of the atmosphere,--Master Paul is unable to raise it."

"He would be able to raise the gla.s.s," said Miss Miette, in a questioning tone, "but he cannot lift the air above it?"

"You are exactly right. But you are going to see an experiment which will prove it. First, however, it will be necessary to take away the globe. I am going to ask Miss Miette to turn this b.u.t.ton, which is called the key of the air-pump."

Miette turned the key, and then they heard a whistling sound.

"It is the air which is entering the globe," said Monsieur Roger. "Now Master Paul can take the globe away."

That was true. When Paul took away the globe, Monsieur Roger put in its place the cylinder closed by the bit of bladder. Then he worked the handle of the machine again. As the air was withdrawn from the interior of the cylinder, the membrane was heard to crackle. Suddenly it burst, with a sort of explosion, to the great surprise of Miette and the amus.e.m.e.nt of everybody.

"What is the matter?" said Miette, eagerly.

"The matter is," answered Monsieur Roger, "that the exterior air weighed so heavily upon the membrane that it split it; and that is what I want to show you. The moment arrived when the pressure of the atmosphere was no longer counterbalanced by the elastic force of the air contained in the cylinder. Then that exhausted all the air, and the atmosphere came down with all its weight upon the membrane, which, after resisting for a little while, was torn."

"Is it true, Monsieur Roger," said Miette, "that it is with this machine that you can make smoke fall?"

"Certainly."

"Well, then, won't you show that to us?"

[Ill.u.s.tration]

[Ill.u.s.tration]

CHAPTER XV.

DROPS OF RAIN AND HAMMER OF WATER.

"I am very willing to show you that," answered Monsieur Roger; "but I must have a candle."

Miette ran to the kitchen and succeeded in obtaining that article which was once so common, and which is now so rare, known as a candle.

Monsieur Roger lit the candle and placed it under the gla.s.s globe of the air-pump. Then he asked Paul to make a vacuum. At the end of a few minutes the candle went out. Monsieur Roger then told Paul to stop.

"Why has the candle gone out?" asked Miette.

"Because it needs air. Master Paul has just exhausted the air necessary to the combustion of the candle; but the wick still smokes, and we are going to see if the smoke which it produces will rise or fall."

Everybody approached the globe, full of curiosity.

"It falls," cried Miette, "the smoke falls."

And in fact, instead of rising in the globe, the smoke lowered slowly and heavily, and fell upon the gla.s.s disk of the air-pump.

"Well," said Monsieur Roger, "you see that I was right. In a vacuum smoke falls: it falls because it no longer finds itself in the midst of air which is heavier than it and forms an obstacle to its fall. In the same way the cloud in the sky above the chateau would fall if we could exhaust the air which is between it and us."

"I am very glad that we cannot," cried Miette.

"And why are you very glad?" asked Madame Dalize.

"Because, mamma, I don't wish any rain to fall."

"Does Miss Miette think, then," said Monsieur Roger, "that if the cloud fell rain would fall?"

"Certainly," answered Miss Miette, with a certain amount of logic. "When the clouds fall they fall in the form of rain."

"Yes; but supposing that I should exhaust the air which is between the cloud and us, the cloud would not fall in a rain, but in a single and large ma.s.s of water."

"Why?"