36 THE WATER 



melting ice. The thermometer is immersed in pure 

 ice that has been crushed and washed with distilled 

 water, so that the graduation, o C. or 32 F., is just 

 visible. The vessel containing the ice should have 

 an opening to permit the water formed to run away, 

 and sufficient time should be allowed for the tempera- 

 ture to become steady. The best way is to make 

 readings at intervals, tapping the thermometer lightly 

 before each, until no further change is noticed. The 

 comparison must be carried out in a room with a 

 temperature above freezing-point, otherwise no melting 

 will take place, and the greatest care must be taken 

 to see that the ice is pure ; any dissolved salts will 

 lower the freezing-point. 



The error of the zero-point, if any, will be applied 

 equally to all temperatures in addition to the errors 

 shown on the certificate. 



The rise of the zero-point maybe avoided to a certain 

 extent by using old thermometers, and some makers 

 graduate their instruments only many years after 

 manufacture ; or the special thermometer glasses may 

 be used, such as the Jena glasses 59 III or 16 III. In 

 any case, however, the instruments should be verified 

 at intervals. 



It may be taken as a general rule that it is not 

 possible to estimate the subdivision of a small interval 

 by eye more closely than to one-tenth, though some 

 observers can pass this limit of accuracy with the 

 greatest ease ; it follows, therefore, that no correction 

 need be considered, for hydrographical purposes, 

 which is less tha'n one-tenth of the smallest division 

 shown. 



If the surface temperature alone is to be observed, 

 an ordinary chemical thermometer, divided to | C., 

 is as a rule sufficient, though one more finely divided 



