THERMOMETER. 



25 



it is hot, he easily slips it on. Upon cooling, it contracts, 

 and so draws the fellies firmly and closely together. 



80. There are several kinds of thermome 

 ter. That in common use in this country is 

 called Fahrenheit s, from the name of the 

 person who first made it. It is made of a 

 glass tube (A B fig. 1,) having a small 

 bore, with a bulb (A) at one end, filled with 

 quicksilver, and fastened upon a plate of 

 metal or other substance, which is to be 

 marked with degrees. When it is to be 

 marked, or graduated, the bulb and tube are 

 held in a mixture of melting snow, or of 

 snow or ice and water. The quicksilver 

 within the tube contracts and falls to a cer 

 tain point, where it remains. Just against 

 this point a line is drawn on the plate of the rig. i. 

 frame, and the number 32 (thirty-two degrees) is 

 marked at the end of it. This is called the freezing point. 



The thermometer is then held in boiling water. The 

 quicksilver expands and rises till it reaches a point at 

 which it remains stationary. Against this point a mark 

 is drawn on the plate, and the number 212 (two hundred 

 and twelve degrees) is made. This is called the boiling 

 point. The space between the freezing and boiling points 

 is divided into 180 equal parts, called degrees. 



The space below the freezing point is divided into parts 

 of this same length, down to the bulb. A thermometer, 

 to be used to measure extremes of cold and heat, should 

 be long enough to extend from 40 or 50 below the 

 freezing, to a few degrees above the boiling point. But 

 as this requires a long tube, instruments for common use 

 are marked up to only 140 or 150. 



