THE BEASON WHY. 15] 



* And God made a wind to pass over the earth." G ENESIS Tin 



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 ISOlbs. ; 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. 



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 atmo- 

 sphere. 



666. WTiy 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 trcde winds f 



