REPORT ON GEOPHYSICAL LABORATORY 1 87 



ature. It would be possible to maintain any room at any desired 

 temperature for any desired period of time consistent with good 

 lighting and ventilation. Such conditions cannot be fully realized. 



Magnetic Disturbances. 



Magnetic observations are so subject to disturbances that in prac- 

 tice it is found needful to provide for them in separate buildings, 

 free from iron and as remote as possible from industrial establish- 

 ments. For more general laboratories, therefore, purely magnetic 

 disturbances may be left out of account, and iron may be freely 

 employed in construction so far as it does not lend itself to the prop- 

 agation of mechanical vibrations. 



Electrical Disturbances. 



Electrical disturbances are of two orders of magnitude: Trolley 

 lines using an earth current produce serious electrical disturbances 

 at a distance of at least one mile, while trolley lines with a double 

 overhead or underground metallic circuit, as well as carriages deriv- 

 ing power from storage batteries, are innocuous at a distance of only 

 a few hundred feet. 



Two Desirable Constant Temperatures. 



Except in deep subterranean chambers, it is difficult at best to 

 maintain uniform temperatures. Far greater is the difficulty of 

 changing the temperature of a room at will, for a very long time 

 must elapse before the massive masonry of the walls and piers 

 acquires the new temperature. For these reasons it appears inex- 

 pedient to attempt more than two temperatures in any laboratory, 

 except perhaps in one small room. One of these temperatures is 

 the mean temperature of the subsoil, say 9 or io° C. in temperate 

 latitudes, and the other is a comfortable temperature for work, 

 say 20 C. 



Annual Mean Temperature. 



The maintenance of the lower temperature with extremely slow 

 variations of a few degrees is not difficult in cellar or subcellar 

 spaces. It is also possible, as Professor Wiechert has shown, to 

 keep the air in such spaces moderately dry. Mr. Wiechert admits 

 the air to his seismometer house through a galvanized-iron tube, 



