B. TERRESTRIAL PHYSICS AND METEOROLOGY. 103 



such observations have been regularly made for the past 

 3000 years. 



The actual diminution in the annual average height of 

 the water, as deduced by Wex, is, for the different parts of 

 the Rhine, from 6 to 60 inches during fifty years; for the 

 Elbe, at Magdeburg, 17 inches; for the Oder, 17 inches; 

 and for the Donau from IS to 55 inches. Sitzungsbericlit 

 der Akademie der Wissensch., Vienna, LXIX., April 23, 1874. 



PHOTOGRAPHING THE WAVES. 



The various mathematical theories that have been, thus 

 far, elaborated with reference to the movements of vessels 

 upon the waves are only approximate. It is only by ex- 

 perience that we are able to determine to what degree of 

 exactness these theories have arrived, and in order to verify 

 them it is necessary to register the successive inclinations 

 that a wave gives to a vessel. The photograph allows us to 

 obtain the law of these inclinations. Let us suppose that a 

 photographic apparatus, having its axis perpendicular to the 

 diametral plane, be directed toward some point in the hori- 

 zon. We should obtain on the sensitive plate an image of the 

 sea and of the heavens, separated by a horizontal line, which 

 would be the image of the horizon. Let us mark upon the 

 plate the position of this image when the axis of the ap- 

 paratus is upright. If it is then inclined by an angle, i 9 about 

 a horizontal axis parallel to the diametral plane, the image 

 of the horizon will remain parallel to the primitive line, but 

 be displaced by a quantity equal to /, tang, i; f being the 

 focal distance of the objective. Let us suppose now that we 

 place before the sensitive plate a fixed shutter pierced with 

 a vertical slip; the image will be intercepted, except in that 

 part of the plate situated behind the slip. We shall thus 

 have upon the plate a broad band of two different tints cor- 

 responding to the sky and the sea, divided by a segment of 

 the horizontal line. Consequently, if we take an instantane- 

 ous photograph at the moment when the ship is inclined at 

 the angle i, we shall have a segment of the horizon line, 

 and the distance of this line from the primitive horizontal 

 line will <nve the anofle i. In order to realize these condi- 

 tions, it suffices to make the sensitive plate move horizontally 

 with a uniform movement. If, during this movement, the 



