THERMAL METHODS 985 



time each morning, ordinarily before the atmospheric temperature has 

 started to increase. After completion of the readings, the thermometers 

 are moved to another set of holes in which they are left until the next 

 morning when measurements are again made. Generally the thermom- 

 eters should be left in the hole for at least 16 to 24 hours before reading, 

 so as to insure their reaching equilibrium with the ground temperature. 



Overcast skies during part of the observations, rain, snow, wind, and 

 other factors affecting the distribution of temperature must be taken into 

 account for proper correlation of observations when they are extended 

 over a considerable period of time. Consideration must be given to the 

 effect produced by varying coefficients of absorption of the surface at 

 different stations. For example, sand or alkali patches have different 

 coefficients of absorption than a surface covered with vegetation. Also, 

 one must take into account that the distribution of temperature under a 

 clump of shade-trees may be entirely different from that in the sun. 



Great care should be taken to locate points of observation as far as 

 possible from the exposed face of an outcrop or from the edge of a deep 

 ditch filled with water or from any point where a sudden change in elevation 

 can be observed. The field notes should show a complete record of all 

 these local circumstances and, if the location of a station at an unfavorable 

 point is imperative, proper care should be taken to detect changes in the 

 distribution of temperature due to these local conditions by staking 

 several stations in the vicinity of the discontinuity. 



For purposes of mapping and correlation, a network of traverses is 

 surveyed, the configuration of which depends on local conditions and on 

 the particular features of the exploration problem. When the suspected 

 structure has a well-defined strike, the traverses may be parallel lines per- 

 pendicular to the strike, and the stations 100 to 500 feet apart, depending 

 on the rate of change in temperature to be expected. It is good practice 

 to start a survey with a base line on which the stations are only a short 

 distance apart and to determine the best distance between stations from 

 the results obtained on this traverse. The measurements on this base line 

 are repeated from time to time as the survey progresses to determine 

 whether the annual variation has any influence on the observations. 



Special care should be taken that the connections between thermocouple 

 and galvanometer are clean and tight. Plugs of the ordinary type generally 

 do not meet these requirements. Care should be taken also that the thermo- 

 couple and the galvanometer are always connected in the same way. The 

 small difference in temperature between the junctions makes it possible to 

 introduce appreciable errors by reversing terminals. Spurious thermo- 

 electric potentials are the greatest source of error in obtaining the field data. 



The Age of the Earth. — As a point of interest, the determination of the age of 

 the earth through geothermal methods may be mentioned. 



The problem has been attacked in various ways. One method divides it into two 



