148 THE ART OF PROJECTING. 



either of the methods described upon pages 31 or 32, 

 the movement of the needle can be noted in degrees. 

 If the needle is too short to reach the numbers upon 

 the glass, it can have a fine straight bristle made fast 

 to its ends with a little mucilage. 



With the thermo-pile connected with the galvanom- 

 eter, the sensitiveness of the former may be shown by 

 presenting the hand to one face of it, or it may be 

 breathed upon or blown upon with a common hand 

 bellows. Let fall a drop of water, of ether, and of 

 alcohol upon the face. The evaporation cools it. 



The heat generated by percussion may be exhibited 

 by providing a leaden bullet which should have at first 

 the same temperature as the thermo-pile, which may be 

 known by putting it upon the pile, handling it with a 

 pair of small tongs. It should not move the needle. 

 Then strike it once with a hammer so as to indent it 

 considerably, and with the tongs quickly put it again 

 upon the face of the pile. It will indicate a higher 

 temperature. 



The heat generated by friction may be shown by 

 rubbing a stick upon the floor and then bringing it to 

 the pile as in the other case. 



See TyndalPs work on Heat for a method of show- 

 ing heat from the crystallization of sodium sulphate. 

 The same thing may be shown with the air thermom- 

 eter sunk into the solution, which may be projected 

 with lantern or porte lumiere by preparing the solution 

 in a beaker, fixing the air thermometer in it with a drop 

 of colored water in it, and projecting the whole upon 

 the screen by means of a large lens. The crystalliza- 

 tion itself will be seen, as well as the manifested heat, 

 when it reaches the bulb of the thermometer. 



Mix in a test-tube resting upon the face of the ther- 



