ON THE RATE OF INCREASE OF UNDERGROUND TEMPERATURE. 



45 



recording apparatus described in the 1869 Report, and the correction to 

 reduce from the actual depth to the depth of 1000 feet is made by- 

 allowing -018 of a degree per foot, this being the mean rate of increase 

 found by observation (see Report for 1871). The above table shows that 

 the entire range of the corrected temperatures at 1000 feet is less than 

 half a degree, and that the departure from the mean exceeds a tenth of 

 a degree on only seven occasions out of twenty-nine. Mr. Symons has 

 directed close attention to those readings which differ most from the 



mean, but has not yet been able to explain the circumstances on which 

 they depend. The maximum elongation of the wire has been 17 feet, and 

 this gives a correction of '31 of a degree. The gradual accumulation of 

 mud at the bottom would account for a gradual change of temperature 

 always in the same direction, if such had occurred (which is not the case), 

 but will not account for alternations of rise and fall such as the table 

 exhibits. 



One of the Committee's slow-acting thermometers has been supplied 

 (at his own expense) to Professor John A. Church, of the Ohio State 



