CHANGES IN THE CIRCULATION. 303 



village. M. Berthier, the chemist, has estimated that the heat fur- 

 nished daily by these springs equals that produced by the combustion 

 of more than four and a half tons of coal, sufficient to comfortably 

 warm the houses and even the streets.* 



Now that the rapid exhaustion of coal-mines forces man to seek 

 the precious combustible at increasingly greater depths, it interests us 

 to know the extreme limit of accessible depth. The report of the Eng- 

 lish commission of inquiry contains very complete data on this point.f 

 The sole cause, says the report, that can place a limit on the practi- 

 cable depth of mines is the elevation of temperature. In the English 

 mines the temperature is of marked uniformity to the depth of about 

 15 metres, viz., 10 Cent. From that depth the heat increases at the 

 rate of 1 to 37 metres, so that at a kilometre of depth it attains blood- 

 heat. This heat hinders the mining operations by heating the air that 

 is artificially made to circulate in the mines, and rendering its effect 

 insignificant. Regarding the question of the highest temperature man 

 can work under without danger to health, the evidence gathered by 

 the commission exhibits some extraordinary cases, but such of them 

 as were verified were found to be exaggerated. Competent authori- 

 ties are, however, united in maintaining that steady labor is impos- 

 sible, in humid air, at a temperature approaching 37. Heat is better 

 supported in dry air. Now, as the deepest mines are generally the 

 least humid, we ought, with the powerful means of ventilation avail- 

 able in these days, to certainly reach a depth of 1,200 metres. We 

 may perhaps go even deeper, thanks to the system of atmospheric 

 shafts recently introduced at Epinac by a French engineer, M. Z. 

 Blanchet. By means of a pneumatic tube the cars are propelled and 

 a thorough ventilation is secured at the same time. By developing 

 this system, very deep deposits can doubtless be reached. 



[To be continued.] 



CHANGES OF THE CIRCULATION DURING CERE- 

 BRAL ACTIVITY. 



By CHAELES SEDGWICK MINOT, S. B., S. D. 



DR. ANGELO MOSSO, the distinguished Professor of Physiology 

 at the University of Turin, has made some observations on the 

 physiology of the brain which, for novelty and interest, have been 

 equaled by but few recent scientific discoveries ; for his researches lie 



* Elisee Reclus, " La Terre," vol. i, p. 239. 



f See in the " Revue " for October 1, 1876, an article on the " Coal Production of 

 England and France." 



