NATURAL PHILOSOPHY. 201 



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reflected rays from the surface by the brims of our tarpaulings, we -\vatched 

 for the disappearance of the plate as it was slowly let down. The varying 

 points between which it disappeared from the vision of all, and where all 

 could see it, never were farther separated than the length that the line 

 could be lifted or let down by a reach of the arm not over four feet. 

 The water varied thermometrically from 40 to 85 



The results proved, that the lowest degree of temperature gave the 

 shortest line of visibility ; and at the point where the water was the warm- 

 est, there we saw the plate at the greatest depth. On two occasions we 

 saw the plate when it was 25 fathoms below the water's surface, and on 

 one of these the water was at 85. On these occasions, all noticed the 

 extraordinary clearness of the water. To lie in the boat and look down, 

 was like looking down from the mast-head. Objects were as clearly 

 defined to a great depth. On this occasion I tried if a contrast of colors 

 would increase the visibility. I so placed the plate upon the pot that a 

 periphery of the black surface surrounded the white plate, but it made no 

 difference at all. 



Capt. Glynn. thought that the maximum of visibility under water, under 

 the most favorable circumstances, is twenty-five fathoms. But between the 

 highest and lowest points of visibility, which corresponded with the highest 

 and lowest points of the water's temperature, there were great variations, 

 which showed no direct correspondence between the temperature and 

 the line of visibility. 



At the mouth of the Mississippi we find the water no more transparent 

 than so much muddy water. The farther we get from the points where 

 earthy matters in large quantities are washed in, the clearer the water is. 

 Xow the Pacific, like the Atlantic, is a great whirlpool a tide flowing 

 entirely around its circumference. In latitude 20 on the west side of the 

 Pacific, farthest removed from all stormy quarters, and where the ocean 

 is stillest, we found the greatest transparency of water. Off Cape Horn, 

 where eternal storms drive up the dirt torn by glaciers and icebergs from 

 the regions around the Pole, the water was exceeding turbid. With the 

 thermometer at 68, we got only ten fathoms of visibility. 



IMPROVEMENTS IN PHOTOGRAPHY. 



The interesting and beautiful science of sun-painting is making rapid 

 and curious advances. A large bridge is being built over the Paver Volga 

 by command of the Czar, who was so impatient to have it completed that 

 he made frequent long expeditions to the works, that he'might use Nature's 

 own stereoscoptics with the enkephalic images implanted in his own imperial 

 "camera obscura." The architect, however, has now made matters more 

 pleasant probably to all parties, by preparing, twice a month, a pair of 

 stereoscopic images of the works as they progress, which he forwards to the 

 Emperor, who sits in his own study, and sees through this new and curi- 

 ous sort of spyglass how they are getting on at the Volga. Another nev/ 

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