660. Why will not beer run out of the tap of a cask until a spile has been driven in at the top?
Because the pressure of the air upon the opening of the tap counteracts the weight of the beer. But when the spile is driven in, the air enters at the top, and counteracts its own pressure at the bottom.
661. Why does a cup in a pie become filled with juice?
Because the heat expands the air, and drives nearly all of it out of the cup. When the pie is taken out of the oven, and begins to cool, air cannot get into the cup again, because its edges are surrounded by juice. A partial vacuum, therefore, exists within the cup, and the pressure of the external air forces the juice into it.
662. Does the cup prevent the juice from boiling over?
No. So long as the heat exists, the cup remains empty; and as it occupies space, the air is driven out of it, into the pie, it rather tends to force the juice over the sides of the dish. It is only when cooling that the juice enters the cup.
663. Why can flies walk on the ceiling?
Because their feet are so formed that they can form a vacuum, under them; their bodies are therefore sustained in opposition to gravitation by atmospheric pressure.
664. How did Mr. Sands perform the feat of walking across the ceiling?
By having large discs of wet leather attached to his feet, so that when they were placed upon a smooth surface, the air was excluded, and when he allowed his weight to act upon one of the discs, it formed a hollow cup and a vacuum. By forming a vacuum of only twelve square inches he gained a pressure of 180 lbs.; this being more than his weight he could accomplish the feat with no other difficulty than that of remaining in an inverted position. The air was admitted underneath the discs by valves, which were closed by springs, which being pressed by the heels of the performer, let in the air, and set the feet free.