THE BEASON WHY. 187 



He shall turn the heart of the fathers towards the children, and the heart of 

 the children to their fathers." MALACHI iv. 



The friction of the air, the friction of the earth, and the 

 attraction of gravitation, which tends to bring all bodies to a 

 state of rest. 



823. Why do the stripes upon a marble disappear when it 

 is spun with great velocity ? 



Because, as in the case of the 

 humming-top, the different parts 

 of the surface are brought so 

 HI \ ' rapidly in succession to the 

 fE^r sight, that they deface or con- 

 fuse the impressions upon the 

 -?L retina. 



824. Why are rings most 

 perceptible at the opposite 



Fig.28.-MA B BLE SPIKNIKaKAPIDLT. V ^ ^ Q/ fa ^^ ? 



Because the point, or pole, upon which the marble spins, and 

 that which corresponds to it, on the upper surface, travel less 

 rapidly than the central portions, which being of a larger 

 circumference, pass through a greater amount of space, in the same 

 period of time. The stripes at the poles of the marble, are, therefore 

 visible, while those at its equator are imperceptible. (See 522.) 



CHAPTER XXXIX. 



825. Why are soap-bubbles round? 



Because they are equally pressed upon all parts of their surface 

 by the atmosphere. 



826. Why are bubbles elongated when being blown ? 

 Because the unequal pressure of the current oj breath by which 



they are being filled, alters the relative pressure upon the outer 

 surfaces. 



827. Why does the bubble close, and become a perfect 

 sphere, wlien shaken from the pipe? 



