220 THE POPULAR SCIENCE MONTHLY. 



Both causes of accident can be avoided by descending and ris- 

 ing 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 quar- 

 ter 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 or- 

 der ; a book would not be sufficient to describe my zoological obser- 

 vations. 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 Amphias- 

 tre 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 div- 

 ing 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. 



