EXAMINATION OF THERMOMETERS. 



369 



change which takes place in the first years after manufacture has ceased, 

 the freezing-point which is then determined is called the permanent 

 freezing-point, and is the zero of the Centigrade scale, or 32 of the 

 Fahrenheit scale. If we heat the thermometer to the boiling-point 

 of water, and then immediately cool it and immerse it in melting ice, 

 we shall obtain another point on the thermometer scale which we may 

 call the temporary freezing-point, because it will gradually approach 

 the permanent freezing-point, and after a few months, if the instrument 

 is not again heated, it will finally coincide with it. The difference 

 between the permanent and temporary freezing-points is usually about 

 three fourths of a degree Fahrenheit, and, so far as now known, remains 

 constant for the same thermometer. 



5. The boiling-point of water at the level of the sea, and with a 

 barometric pressure of 760 mm. = 29-922 inches in the latitude of 45, 

 is the second point in the thermometer scale to be fixed. To do this 

 the thermometer is exposed to the steam of pure water, and, from the 

 observed height of the barometer, the known elevation and latitude of 

 the place of observation, the true boiling-point is computed from the 

 observed one, and the 100 C. or 212 F. is thus fixed. 



6. Having thus located the freezing and boiling points of a stand- 

 ard thermometer, the intermediate points are to be fixed by dividing 

 the scale so that at every part the length of 1 shall measure an equal 



Fig. 1. 



volume of mercury. At least, this has been the usual procedure, and 

 for ordinary standards perhaps it is the most convenient. For stand- 

 ards to be used in the highest class of work it would be better to 

 graduate the distance between and 100 C. into one hundred equal 

 parts, and then allow the observer to accurately determine the value 

 of the corrections at each degree. Indeed, it is preferable for many 



TOL. XTIII. 24 



