HEAT PHENOMENA. 261 



The smallest depth, however, of that section of the tunnel, 10,000 feet long, 

 for which the temperatures are plotted is above 1,000 feet, and for the last 

 5,500 feet of the tunnel the average depth below the surface is about 1,500 

 feet, with comparatively small surface variations. When it is considered 

 that the annual variation of temperature commonly ceases to be perceptible 

 at a depth of 100 feet, it appears that the irregularities of temperature in the 

 Sutro Tunnel due to the character of the surface topography, above this last 

 5,500 feet at least, must be insensible. The variations from the exponential 

 locus are, no doubt, due to the character of the rock, which, as is indicated 

 in the section (Atlas-sheet VI.), shows alternate belts of greater and less 

 decomposition. Rock temperatures would have been preferred to water 

 temperatures had they been recorded, but such was not the case. 



Conditions in the laterals. — Rock teuiperaturcs havc been taken from time to 

 time in the north and south lateral branches of the Sutro Tunnel, but in 

 large part these branches pass close to mines which have been worked for 

 years to a much lower level than that of the tunnel. The mean of the 

 observations taken in the south branch as "far as the Imperial ground is 

 almost exactly the same as that of the observations in the north branch as 

 far as the Ophir, 113° or 114°, thus confirming the fact, already well known, 

 that within the limits indicated the mines are all hot, and, on the whole, 

 pretty nearly equally so. The north branch near the 02)hir is three or four 

 degrees hotter than might have been anticipated from the observations in 

 the main adit, and the Aljiha and Exchequer claims are nearly as hot. Such 

 variations are certainly to be expected. They may indicate local peculiar- 

 ities of structure, such as the presence of diagonal fissures leading to the 

 Lode, or very possibly the prevalence of slightly higher temperatures 

 throughout the regions lying to the north and south of the main tunnel. 



Regularity of the Forman curve. — A compaHsou of thc diagrams sliows that the 

 observations in the Forman shaft reveal an increment not greatly more 

 irregular than those observed at the Rose Bridge colliery, and at Sperenberg. 

 In view of the local character of the abnormal temperatures near the Com- 

 STOCK this fact is remarkable. 



Results. — The five lines of temperatures near the Lode, form a tolerably 

 complete thermometric survey, and justify conclusions of a definite char- 



