2O6 METHODS IN TEACHING 



as follows : Take a rod of iron, copper, or brass, about two 

 feet long, and place it in a horizontal position with one end 

 against any solid object. Let the other end touch the short 

 arm of a lever which acts as an indicator. Now heat the 

 rod, and it will be noticed that it expands and moves the 

 indicator. Why does the blacksmith heat wagon tires be- 

 fore putting them upon the wheels ? Why are small spaces 

 left between the ends of rails on a railway ? Why does the 

 column of mercury rise in warm weather and fall in cold? 

 In connection with the expansion of gases and liquids by 

 heat, air and water currents may be taken up. For the 

 study of air currents there should be pro- 

 Water Currents v *d e d a glass-sided box, air tight, except 

 for openings which may be closed when 

 necessary. Such a box may be eight or ten inches high, 

 eighteen inches long, and from six to ten inches wide. The 

 sides should be of glass, puttied in, and a door at one end 

 should be as nearly air tight as possible. Augur holes, of 

 a size that they may be closed by means of large cork stop- 

 pers, should be bored in the top near each end, and som< 

 may also be bored at different heights in the end opposite 

 the door. 1 A burning candle will act as a stove in the 

 thus made. If the box is fairly tight and all openings ai 

 closed, the candle flame will go out in a few moments aftei 

 it is lighted. Why? Now open the holes at the top am 

 place the lighted candle under one of them. Test the din 

 tion of the air currents by holding smoking punk or tou< 



a The glass-sided box described above is very satisfactory for tl 

 work indicated, but it is not indispensable. A pasteboard box witl 

 out glass may be used, although it will be less satisfactory. 



