Page:Popular Science Monthly Volume 38.djvu/232

220 Both causes of accident can be avoided by descending and rising slowly. For this reason a steel chain may be used as a ladder, to be let down to the depth the diver has reached, by the aid of which he can stop at will while coming up. But the question of time comes in to limit the depth which it is possible to reach. If we allow three quarters of an hour for a diving excursion, a quarter of an hour will be required to descend below thirty metres, and as long to come up; so that only a quarter of an hour is left for staying on the bottom.

Of the scientific observations which I have been able to make with the diving-dress, I will speak only of those of a physical order; a book would not be sufficient to describe my zoölogical observations. When the water is transparent and the sun shining, we can, looking down from the boat, distinguish the bottom to about twenty metres; but for that the surface should be perfectly smooth. I have had fixed in the bottom of my yacht Amphiastre a light-port with a very thick glass. By darkening the cabin we can see through it clearly, farther than twenty metres, even when the surface of the sea is troubled. Seen thus from above, the bottom of the sea always looks flat. All the visible parts are equally lighted, and the appearance of relief is naturally destroyed by the absence of projected shadows. In going down in the diving apparatus, we are astonished at perceiving that the ground, which appeared nearly uniform, is really bristling with rocks and hollowed by deep valleys. The shadows are now visible, because the light coming from above, the parts under the projections of the rocks and the tufts of sea-weed are in the dark. If the diver looks up from the bottom through the frontal glass of his casque, he will see a great light, circular space that may be regarded as the base of an inverted luminous cone, of which the spectator's eye occupies the tip, and the apical angle of which is about 62° 50'. Beyond this circle the surface looks dark, presenting precisely the aspect of the sea as seen when looking down into it from the boat. The sky and objects in the air are visible only within the limits of the luminous circle. The borders of this circle are always more or less indented, for the surface is never perfectly quiet. The sunbeams are dimmed and come down in dancing showers as we see them in a room on the edge of the water when the blinds are drawn down, and the rays, reflected from the mobile surface, shine upon the ceiling of the room.

The decrease of density of the sun's rays is very rapid, and they are almost completely diffused at thirty metres. As the sun declines toward the horizon, a darkness suddenly comes on which has sometimes caused me to ascend very speedily, in the belief that night had fallen. Coming out of the water, I was astonished to find myself immersed in the rays of a sun not yet near setting.