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_. [Verse: "And God made a wind to pass over the earth."--GENESIS VIII.] 665. _Why is it difficult to strike limpets from rocks?_ Because they have the means of forming a _vacuum_ under their shells, and are pressed on to the rocks by the weight of the atmosphere. 666. _Why can snails move over plants in an inverted position?_ Because they form a _vacuum_ with the smooth and moist surfaces of their bodies, and are supported by atmospheric pressure. CHAPTER XXXI. 666. _What is wind?_ Wind is air _in motion_. (_See_ 234.) 667. _What are the velocities of winds?_ A _breeze_ travels ten feet in a second; a _light gale_, sixteen feet in a second; a _stiff gale_, twenty-four feet in a second; a _violent squall_, thirty-five feet in a second; _storm wind_, from forty-three to fifty-four in a second; _hurricane_ of the temperate zone, sixty feet in a second; _hurricane_ of the torrid zone, one hundred and twenty to three hundred feet in a second. When wind flies at one mile an hour, it is scarcely perceptible. When its velocity is one hundred miles an hour, it tears up trees, and devastates its track. 668. _What are trade winds?_ Trade winds are vast currents of air, which _sweep round the globe_ over a belt of some 12,000 miles in width.