OBJECTS.] -INTRODUCTORY. 49 



bulb is placed in boiling water ; and another mark 

 at the point to which it sinks when the bulb is in 

 melting ice ; and the space between the two marks is 

 divided into 180 equal parts, each of these parts is 

 what is called a " degree" in the thermometers ordinarily 

 used in this country (called Fahrenheits). And if 

 the boiling-point is counted as 212 the freezing-point 

 must be 3 2 (2 12 32 = 180). With the same amount 

 of heat the fluid in the tube always stands at the same 

 degree, and hence the instrument measures tem- 

 perature. 



* That hot water is lighter than cold is easily seen 

 when a bath is filled from two taps, one of hot and 

 one of cold water, which run at the same time. Un- 

 less care is taken to stir the water, the top of the bath 

 will be very much hotter than the bottom. Thus, an 

 imperial pint of water weighs a pound and a quarter 

 only at a certain temperature or degree of warmth, 

 namely at 62 ; if it is made hotter its volume increases, 

 and therefore its specific gravity diminishes. 



It was for this reason that in 22 the weight of the 

 same volume of water was said to be constant under 

 the same conditions ; and, of course, the same 

 qualification must be borne in mind when we speak 

 of the wefght of a cubic inch of water being about 

 252 and a half grains. Its weight is in fact 252*45 

 grains only when Fahrenheit's thermometer stands at 

 62 but as this is the temperature of ordinary mild 

 weather, and the expansion or contraction of water 

 for a degree about this temperature amounts to less 

 than -g-^j-th of its volume, the weight of a cubic inch 

 may for all practical purposes be taken as 252 and a 

 half grains. 



