Geology and Natural History. 487 



terminals consist of aluminum mirrors. The rays impinge upon a 

 Y-shaped reflector of platinum which is placed in the middle of 

 the tube. The effects obtained with this tube are very remark- 

 able. I have been able to trace the outline of the heart and of 

 the liver and to see all the ribs. The shadow of the hand can be 

 seen through two oak doors at a distance of fifteen feet ; and 

 sensitive plates are fogged through brick walls a foot thick at a 

 distance of fifteen feet. It has been found possible to construct 

 tubes which are much better able to stand the strong excitation of 

 a Tesla coil than hitherto. j. t. 



9. Diminution of the intensity of Sound with the distance. — 

 Various attempts have been made to determine the diminution of 

 the intensity of sound with the distance especially by Vierordt 

 and by Wien. The former experimented with the sound pro- 

 duced by weights falling on plates from different heights. He 

 reached the result, that the sound diminished according to a 

 linear relation with the distance. Wien measured the diminution 

 of the sound intensity by means of determination of the ampli- 

 tude — and stated the law that the intensity of sound diminishes 

 with the square of the distance — the law is modified by frictional 

 resistances. Karl L. Schaefer takes up the question anew, by 

 the following method. A watch is placed at a certain distance 

 from one ear and a telephone excited by a suitable interrupter is 

 placed at a certain distance so as to just overcome the slightest 

 ticking of the watch. Then the observation is repeated with a 

 different position of the telephone and from a combination of the 

 observations, it was seen that Vierordt's results were unreliable. 

 The sound intensity in the neighborhood of the telephone dimin- 

 ishes more slowly than the square of the distance. With increas- 

 ing distance the diminution increases, until the quadratic diminu- 

 tion is approximately reached and afterwards even exceeded. — 

 Ann. der Physik unci Chemie, No. 4, 1896, pp. 785-792. j. t. 



II. Geology and Natural History. 



1. Fifteenth Annual Report of the United States Geological 

 Survey. — This report has just been delivered by the Public Prin- 

 ter. It is a handsome volume of 755 pages and 48 plates, and 

 contains, besides the administrative reports of the Director him- 

 self and of chiefs in charge of work, the following special papers : 

 " Preliminary Report on the Geology of the Common Roads of 

 the United States," by Prof. N. S. Shaler; "The Potomac For- 

 mation," by Prof. L. F. Ward ; " Sketch of the Geology of the 

 San Francisco Peninsula," by Andrew C. Lawson; "Preliminary 

 Report on the Marquette Iron-bearing District of Michigan," by 

 Prof. C. R.-Van Hise, W. S. Bayley, and H. L. Smyth ; and " The 

 Origin and Relation of Central Maryland Granites," by C. R. 

 Keyes, with an " Introduction on the General Relations of the 

 Granitic Rocks in the Middle Atlantic Piedmont Plateau," by 

 the late Prof. G. H. Williams. 



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