626 THE POPULAR SCIENCE MONTHLY. 



and allowed to cool. The mercury in this position closes the 

 upper end of the tube, and as the cooling proceeds it is sucked 

 through the capillary opening and falls, drop by drop, into the 

 bulb below. The process is repeated, if necessary, until the lower 

 bulb is filled with mercury. The thermometer is then heated. 

 The mercury expands, driving out all the air and filling both bulb 

 and tube. The temporary bulb is now removed, and the open 

 end of the tube is closed before the blowpipe. The thermometer 

 is ready for calibration. 



The bulb is buried in cracked ice, from which the water is al- 

 lowed free drainage. When the mercury no longer contracts, a 

 mark is made on the tube at a level with the mercury. This is 

 the freezing-point of water, on the centigrade scale, or 32 on 

 the Fahrenheit. Reaumur's scale, with the freezing-point at 

 and the boiling-point at 80, although so extensively used in Ger- 

 many and Russia, is seldom seen in this country. The thermom- 

 eter is then transferred to a bath of boiling water. The mercury 

 quickly rises, and soon again becomes stationary. The tube is 

 marked for the second time. This is the boiling-point of water, 

 100 centigrade, or 212 Fahrenheit. As the temperature of the 

 boiling-point varies with the atmospheric pressure, the barometer 

 must be read and a corresponding correction made, or else a stand- 

 ard thermometer must be kept in the bath, and the marking made 

 in harmony with that. These two points determined, the opera- 

 tion of making a thermometer is almost completed. It has now 

 only to be marked. 



The tube is dipped into a bath of melted beeswax, and as soon 

 as the thin layer of wax hardens it is taken to the dividing-en- 

 gine. The space between the freezing and boiling points is here 

 divided off into 100 divisions if the centigrade scale is to be 

 employed, or into 180 divisions if the Fahrenheit be used. Every 

 tenth line is made somewhat longer than the others, and is the 

 only one marked. The marking is done on a machine constructed 

 after the order of a pantograph. The waxed tube is laid on a 

 small sliding platform and secured to its bed by a few drops 

 of melted wax. A sharp stylus is then brought to bear upon the 

 point where the marking is first wanted. The movement of the 

 stylus is controlled by a long lever, whose own movement is, in 

 turn, controlled by the action of a second stylus. This is made to 

 pass over the desired figures cut in brass on a lower platform of 

 the machine. The action of the system of levers is to reduce the 

 motion of the upper stylus, and consequently the size of the fig- 

 ures traced through the wax. In this way accurate marking is 

 secured on a sufficiently small scale. The tube, thus lined and 

 marked, must now be subjected to the action of hydrofluoric acid. 

 A solution of the acid in water, to which some alkaline salt has 



