REVIEW. 441 



59. (a.) Why does a certain piece of glass look red when it is 

 held between a lamp and the eye ? (6.) Why does it look red when 

 the lamp is between the glass and the eye ? (c.) Explain the suc- 

 cession of colors in the rainbow, (d.) What three classes of rays in 

 a sunbeam ? 



60. (a.) Describe the human eye as an optical instrument. (6.) The 

 opera-glass, (c.) The terrestrial telescope. (rf.) The stereoscope. 



61. (a.) Explain polarization of light by absorption. (6.) By 

 reflection. 



62. (a.) Explain the action of the siphon. (6.) Find the volume 

 of a balloon filled with hydrogen that has a lifting power of 440 Ibs. 

 (sp. gr. of air 14.42. One liter of hydrogen weighs .0896^.) 



63. (a.) The barrel of an air-pump is that of the receiver ; find 

 the tension of the air in the receiver after 8 strokes of the piston, call- 

 ing the normal pressure 15 Ibs. and disregarding the volume of the 

 connecting pipes. (&.) A stone let fall from the top of a cliff was 

 seen to strike the bottom in 6^ seconds ; how high was the cliff ? 



64. (a.) A ship passing from the sea into a river, discharges 44800 

 Ibs. of cargo, and is found to sink in the river to the same mark as 

 in the sea. The sp. gr. of sea-water being 1.028, find the weight of the 

 ship and cargo. (6.) A body weighing 12 Ibs. (sp. gr. = |,) is fastened 

 to the bottom of a vessel by a cord. Water being poured in until 

 the body is covered, find the tension of the cord. 



65. (a.) If the intensity of gravity at the moon be ^ of that at the 

 earth, find the length of a seconds pendulum at the moon, the length 

 of one at the earth being 39.1 inches. (6.) Find the maximum weight 

 that can be supported by a hydraulic elevator connected with a 

 reservoir, the area of the piston being 24 sq. in. and the reservoir 

 being 170 ft. above the cylinder, (c.) The difference between the 

 fundamental tones of two organ-pipes of the same length, one of 

 which is closed at the top, is an octave. Explain why. 



