492 ANNUAL RECORD OF SCIENCE AND INDUSTRY. 



The hypothesis upon which M. Dumont proceeded in under- 

 taking his labors, so satisfactorily accomplished, was that 

 there exists under the gravel and sands of the Rhone, and 

 under the course of all waters of an analogous nature, a vol- 

 ume of water perfectly clarified (really an inferior and sub- 

 terranean river), and that these gravels, etc., are genuine fil- 

 ters, which cleanse themselves by a double process, their 

 product being always the same. The labors executed by the 

 author at Lyons and elsewhere have proved to him the cor- 

 rectness of these views, and enabled him to establish the true 

 principles which should be taken into consideration in the 

 execution of similar labors. These are, first, to give the pref- 

 erence to lateral galleries instead of filtering basins; second, 

 to bring these galleries as near as possible to the principal 

 current of the river ; third, to give these galleries the largest 

 interior diameter possible ; and fourth, to build the abut- 

 ments up to the level of the low water-mark only, and make 

 the layer of the filtering frame-work in the form of a cradle. 

 6 B.June 3, 1872, 1451. 



INDICATION OF HEATING BY FRICTION. 



The history of science is filled with illustrations of the fact 

 that abstract discoveries, apparently of little practical bear- 

 ing, are often turned to very important economical account. 

 A new instance of this is shown in the recent discovery, by 

 Mensel, that certain double iodides, in a strong degree, and 

 other substances"to a less marked extent, possess the property 

 of readily changing color upon the application of a compara- 

 tively slight degree of heat. 



One of these applications is by Professor Mayer, who em- 

 ploys a double iodide of copper and mercury for obtaining a 

 precise method of tracing the progress and of determining 

 the boundary of a wave of conducted heat ; and the same 

 gentleman suggests that this and other sensitive compounds 

 be painted upon the lyillow blocks, and other parts of a ma- 

 chine liable to injurious heating from friction. It will enable 

 the engineer to determine the temperature of the moving 

 parts of his apparatus, and to be on the watch for any injuri- 

 ous effect of heating by friction. 



The iodide referred to, within the limits of the freezing and 

 boiling points of water, changes from a brilliant carmine red 



