4 THERMOMETER. 



necessary to wipe it rapidly, and not to record the degree until 

 the instrument has been allowed to acquire the true temperature 

 of the air. 



Verification. Verify the zero point, at the beginning and end 

 of winter. For this purpose, fill a vessel with snow, immerse the 

 bulb of the thermometer in the middle of it, so as to be sur- 

 rounded on every side by a layer of several inches of snow, 

 slightly pressed around the instrument. The stem must be 

 placed exactly perpendicular, and covered with snow as far up 

 as the freezing-point on the scale. Let it stand so for half an 

 hour or more, and then read it, taking great care to place the 

 eye at the same height as the summit of the mercurial column. 

 If the top of the column does not coincide with the freezing- 

 point of the scale, the exact amount of the difference must be 

 ascertained, and the correction immediately applied. At the 

 same time enter in the journal, under its -appropriate head, the 

 day on which the experiment is made, its quantity, and the hour 

 at which the application of it was commenced. [It is neces- 

 sary to add, that since the zero point of the thermometer is 

 not that of the temperature of snow as it is frequently found 

 when exposed to the atmosphere, but that of melting snow, the 

 experiment must be made in a place above the temperature of 

 freezing. Instead of snow, pounded ice may be employed.] 



[Green's thermometers have an arrangement by which the tube 

 can be slipped down the small quantity necessary to correct for 

 this change. The end of the tube is fitted into a small plate of 

 German silver, and this fastened by a screw to the scale. If, on 

 testing the thermometer, the mercury be found to stand above 

 32, free the screw one or two turns without taking it out, and 

 push down the plate the necessary amount to bring the mercury 

 to coincide. The thermometer must be handled with great care 

 in making this adjustment, and it may be well, for additional 

 security against accident, to loosen all the screws which fasten 

 the bands around the tube; it will then slide in them more freely. 

 After completing the adjustment, they may again be set mode- 

 rately tight. The object of this adjustment being only to avoid 

 the trouble of making a correction, it is not advisable to attempt 

 it, if the observer thinks that he risks, in so doing, the safety of 

 his instrument. As the tubes of these standard thermometers 

 are kept for a considerable time before fixing the zero point, in 



