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PROCEEDINGS OP THE AMERICAN ACADEMY 



TABLE I.— Showing Change of Zero Point. 



The second 100° reading was taken after boiling for some time. 



It is seen that the zero point is always lower after heating, and that 

 in the limits of the table the lowering of the zero is about propor- 

 tional to the square of the increase of temperature above 25° C. 

 This law is not true much above 100°, and above a certain tempera- 

 ture the phenomenon is reversed, and the zero point is higher after 

 heating ; but for the given range it seems quite exact. 



It is not my purpose to make a complete study of this phenomenon 

 with a view to correcting the thermometer, although this has been 

 undertaken by others. But we see from the table that the error can- 

 not exceed certain limits. The range of temperature which I have 

 used in each experiment is from 20° to 30° C, and the temperature 

 rarely rose above 40° C. The change of zero in this range only 

 amounts to 0°.03 C. 



The exact distribution of the error from this cause throughout the 

 scale has never been determined, and it affects my results so little that 

 I have not considered it worth investigating. It seems probable, how- 

 ever, that the error is distributed throughout the scale. If it were 

 uniformly distributed, the value of each division would be less than 

 before by the ratio of the lowering at zero to the temperature to 

 which the thermometer was heated. 



The maximum errors produced in my thermometers by this cause 

 would thus amount to 1 in 1300 nearly for the 40° thermometer, and 

 to about 1 in 2000 for the others. Rather than allow for this, it is 

 better to allow time for the thermometer to resume its original state. 



Only a few observations were made upon the rapidity with which 

 the zero returned to its original position. After heating to 81°, the 

 zero returned from — 0°.170 to — 0°.148 in two hours and a half. 

 After heating to 100°, the zero returned from — 0°.347 to — 0°.110 

 in nine days, and to — 0°.022 in one month. Reasoning from this, I 



