HEAT PHENOMENA. 263 



laterals of the tunnel. On the other hand, these equations give for the 

 Sutro Tunnel level (1,865 feet below the datum) temperatures five or six 

 degrees higher than are found at the corresponding points in the adit. This 

 agrees well with the supposition already suggested, that the isothermal 

 surfaces rise somewhat towards the south; but the data are too uncertain, 

 and the rock is too heterogeneous to warrant applications of the equations 

 implying their absolute accuracy. It appears to me, under the conditions, 

 extremely remarkable that the relations of temperature to depth and hori- 

 zontal distance from the Lode are capable of even approximate mathematical 

 expression. 



Practical data. — Wlthiu the belt of country between 2,500 and 3,500 feet 

 from the croppings, the relation .of temperature of the rock to depth is 

 expi'essed approximately by the equation 



t = AO + 0.033 d, 



d being measured fi-om the datum level in feet; and this equation may be 

 expected to hold good, with local fluctuations, for a long distance below the 

 present workings. The equation gives for a tempei'ature of 212° a depth 

 of 5,200 feet The water will be found commonly hotter than the rock, 

 and its temperature also more variable. It is not unlikely to be struck at 

 a boiling heat any time after the 4,000-foot level is passed, and will in all 

 probability be struck short of 5,000 feet. 



inferencesfrom the Sutro curve. — Tlie curve obtained froui the observatious made 

 in the Sutro Tunnel is clearly a conduction curve, and proves that the east 

 country is heated from a surface at or near the Lode. If the Lode is sup- 

 posed to liave assumed its present temperature suddenly, the radius of cur- 

 vature of tliis locus would be a function of the time, and if the coefficient 

 of conductivity of the rock, its initial temperature, etc., and all the condi- 

 tions of radiation from the surface were kilown, the time which has elapsed 

 since the Lode grew hot might be calculated. It is not likely, however, 

 that the temperature of the Lole has always been constant or nearly so, 

 and there is no means of inferring the constants, a definite knowledge of 

 which would be necessary to a mathematical discussion of this jjroblem. 



