In Search of a Son

by William S. Walsh

"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 mass 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 quantity of air above the piston, and yet I could make it go up very easily."

"Certainly, there was above the piston the same quantity 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 glass cylinder, whose upper opening was closed by a bit of bladder stretched taut and carefully tied upon the edges.