OF THE SPECIFIC GRAVITIES OF BODIES. 187 



summer ; as much spirits of nitre, 20 grains ; vinegar 6 LECT. 

 grains, and spring-water 3. Hence it is most profitable V^-N^X. 

 to buy spirits in winter, and sell them in summer, since 

 they are always bought and sold by measure. It has 

 been found, that 32 gallons of spirits in winter will make 

 33 in summer. 



The expansion of all fluids is proportionable to the 

 degree of heat ; that is, with a double or triple heat, 

 a fluid will expand two or three times as much. 



Upon these principles depends the construction of The ther 

 ... . . i 11 j , mometer. 



the thermometer, m which the globe or bulb, and part 



of the tube, are filled with a fluid, which, when joined 

 to the barometer, is spirits of wine tinged, that it may 

 be more easily seen in the tube. But when thermome- 

 ters are made by themselves, quicksilver is generally 

 used. 



In the thermometer, a scale is fitted to the tube, to 

 shew the expansion of the quicksilver, and consequently 

 the degree of heat. And, as Fahrenheit's scale is most 

 in esteem at present, I shall explain the construction and 

 graduation of thermometers, according to that scale. 



First, Let the globe or bulb, and part of the tube, be 

 filled with a fluid ; then immerse the bulb in water just 

 freezing, or snow just thawing; and even with that 

 part in the scale where the fluid then stands in the 

 tube, place the number 32, to denote the freezing point: 

 then put the bulb under your arm-pit, when your body 

 is of a moderate degree of heat, so that it may acquire 

 the same degree of heat with your skin ; and when the 

 fluid has risen as far as it can by that heat, there place 

 the number 97 : then divide the space between these 

 numbers into 65 equal parts, and continue those divi- 

 sions both above 97 and below 32, and number them 

 accordingly. 



This may be done in any part of the world ; for it is 

 found that the freezing point is always the same in all 

 places, and the heat of the human body differs but very 



