ON UNDERGROUND TEMPERATURE. 



183 



tube only rises one foot above the floor, and is plugged with a large ball of 

 felt to prevent external air having free communication with the tube. The 

 exact limits of the various strata are also shown, together with the constant 



A, floor, 10 feet below surface of ground. 

 B, briok-ledge. C, bore-tube, fitting tightly 

 in floor. D, steps leading to entrance door E. 

 G, opening into well, with trap-door. H, 

 beam suporting pulleys, over which pass two 

 cords Q Q,, one leading to tube and the other 

 to well. J, windlass, separately represented 

 in second figure. L, registering-apparatus, 

 with dials M, indicating amount of cord paid 

 out. N, stand of windlass, fixed to brick- 

 work B. R, thermometers for temperature 

 of observing room. O O, thermometers for 

 imdorground temperature. 



depth at which water stands in the tube : this constancy is worth notice ; for 

 whereas in most cases water-levels vary with the rainfall in the districts 

 whence they obtain their supply, the water at Kentish Town has not varied 

 more than six inches during the last ten months, and is very muddy. The 

 diameter of the bore-tube is 8 inches. 



Two thermometers have always been used in these observations, — one 



