/ 



C. GENERAL PHYSICS. 167 



place ; this being based upon the hypothesis that two fluids, 

 of diflerent specific gravity, will long remain separate if kept 

 still. On this account it is often maintained that the car 

 bonic acid gas of an occupied room is in largest percentage 

 near the floor, and that in sleeping at a point somewhat 

 above this the accumulations of the gas will be comparative- 

 ly innocuous. 



Dr. Pettenkofer, however, has recently substantiated the 

 assertions of previous writers, as to the inaccuracy of such 

 views, by a series of experiments made at Marionbad, where 

 a spring throwing off" carbonic acid gas very rapidly has a 

 small wooden house built over it, and into which the gas, 

 containing at least 70 per cent, of carbonic acid, is continual- 

 ly discharging. He finds, as the result of continued and 

 careful investigations under these circumstances, that car- 

 bonic acid gas is diff'used into the atmosphere with the ut- 

 most rapidity ; so much so, indeed, that at a distance of only 

 an inch above the w^ater surface, more than two volumes of 

 the atmospheric air were already mixed with the gas of the 

 spring ; and that, still higher, the diflusion is so complete 

 that the air is quite respirable. 18 .4, J/y 30, 1873, 262. 



THE FLOW OF AVATER IN KIYERS AND CANALS. 



We have already given in some detail the valuable theo- 

 retical results attained by Cialdi in studying the various im- 

 portant problems of hydraulics. In the practical labor of 

 measuring exactly the currents in rivers, a decided improve- 

 ment appears to have been made by D. F. Henry, chief en- 

 o-ineer of the water-works in Detroit, in the invention and 

 application of his telegraphic current meter. By the inven- 

 tion of this instrument, and especially by the extended se- 

 ries of experimental determinations made with it, he has con- 

 tributed materially toward an advance of the science of hy- 

 draulics. 



The principal application of his current meter thus fiir 

 made consists in the measurement of the currents of the St. 

 Clair River, both on the surface of the water and at depths 

 successively increasing to forty-five feet. Henry concludes 

 that in large rivers we may consider the surface velocity in 

 a calm time to be nearly the same as the maximum velocity, 

 while in narrow canals, with vertical sides, it may be con- 



