THE PHOTOPHONE. 361 
mains in the tributary snow-belt, and the result is, that as the snow is unshaded, 
the feeble warmth of early spring sun is admitted, it begins to thaw first slowly, 
but faster as the warmth increases, so that all the snow unshaded is thawed and 
passed into the streams and small rivers in March to find rest in the Mississippi 
the early part of April; and the snow thawing so gradually off the lands cleared 
that it now makes little perceptible change in the river below Keokuk—a fact well 
known to river men. The quarter of the snow-belt yet shaded (but gradually di- 
minishing) gets to the river about the first of May, to be shaded by the Missouri 
about the last of May. This gradual outcome of the snow-water is the chief 
cause of the gradual decrease of the average high water. Next, cause in impor- 
tance is the increase of the cultivation and draining of the land tributary as the 
surface is cultivated, the absorption is increased, and the ditches pass off quickly 
what is not absorbed; and as the streams conveying surplus surface water to the 
river, the lower emptying will keep out of the way of the upper, and the capacity 
of the river where one and a half to two miles wide, as it is below Cap au Gris, is 
sufficient to hold extraordinary rain-floods, as demonstrated in the last. Again, 
as the surface of the ground is cultivated or cleared the evaporation increases, 
which is no small matter in the extent of area that drains into the valley, and, 
lastly, the increase of current. In answer to the theory some persons have, that 
as the timber is cut off the floods are higher, and particularly refer to some rivers 
in Europe, let me say that such rivers head in mountains, in which the main 
channel and tributaries have rapid current, consequently rise and fall very rapidly. 
THE PHOTOPHONE. 
In May, 1878, Mr. Alexander Graham Bell, well known in connection ,with 
the telephone, announced before a scientific society in London, his belief that it 
would be possible to hear a shadow by interrupting the action of light upon sele- 
nium. At the recent meeting of the American Science Association in Boston, 
Mr. Bell read a paper describing at length his experiments in the production and 
reproduction of sound by light, and the invention by Mr. Sumner Tainter and 
himself of an instrument for the purpose. 
The influence of light upon the electric conducting power of selenium 1s 
well known. Mr. Bell found the electric resistence of some selenium cells of 
peculiar construction only one-fifteenth as much in the light as in the dark. It oc- 
curred to him that all the audible effects obtained in the telephone by variation 
of the electric current by sound waves, could also be produced by variations of 
light acting upon selenium; and that with suitable transmitting and receiving ap- 
paratus voices might be conveyed without a wire along a line of light. 
The fundamental idea on which rests the possibility of producing speech by 
the action of light is the conception of what Mr. Bell terms an undulatory beam 
of light in contradistinction to an interrupted beam; meaning by the former a 
beam that shines continuously, but is subject to rapid changes of intensity. 
