IN NATURAL PHILOSOPHY. 43 



The well extends below the influence of the sun, and not 

 deep enough to reach the internal heat of the earth. 



25. Ought the bottom of a tea-kettle to be polished? 



No, since a polished surface would reflect the heat. We 

 need a black, rough, sooty surface to absorb the heat rapidly. 



26. Which boils the sooner, milk or water? 



Milk, because it is so adhesive that the bubbles of steam 

 which are formed at the bottom of the dish cannot easily 

 escape. They therefore pile up on top of each other, and the 

 milk boils over readily. 



27. Is it economy to keep our stoves highly polished? 



The stove-blacking used is a good radiator, but the surface 

 should not be highly polished, as that hinders radiation. 



28. If a thermometer be held in a running stream, will it 

 indicate the same temperature that it would in a pailful of 

 the same water ? 



It will. For the same reason that a thermometer, in the 

 wind, will indicate the same temperature as in the still air. 

 although the former seems to us much colder. 



29. Which makes the better holder, woollen or cotton f 

 Woollen, because it is so poor a conductor of heat. 



30. Which will give out the more heat, a plain stove or one 

 with ornamental designs ? 



The latter, since it has more radiating surface 



3 1 . Does dew fall ? 



No ; it forms directly where it is found. The vapor merely 

 collects on the cold surface. 



32. What causes the " sweating" of a pitcher? 



The vapor of the air condenses on the cold pitcher. It is 

 often a sign of rain, since it shows that the air is full of vapor 

 easily deposited. 



33. Why is evaporation hastened in a vacuum f 

 Because the pressure of the air is removed. 



