_The Transparency of the Sea. _--At a late meeting of the French Academy of Science, M. Cialdi and Father Secchi sent the result of some observations they have made "On the Transparency of the Sea." The experiments were made at the end of April, on board a vessel, near Civita Vecchia, from six to twelve miles from land, and at depths varying from 90 to 300 metres, the sea being perfectly clear and tranquil. Discs of different diameters and colors attached to wires being plunged horizontally under water, showed that the maximum depth at which the largest (a white disc 3-1/4 metres in diameter) could be seen was 42-1/2 metres, the sun being elevated 60-1/4° above the horizon. With a vertical sun the depth of visibility shall be 45 metres. The color of the disc appeared at first a light green, then a clear blue, which became darker as it was lowered, until it could no longer be distinguished from the surrounding medium. Discs of a yellow or sandy color disappeared at less than half the depth of the white discs--that is to say, between 17 and 24 metres. The height of the sun and the clearness of the sky greatly influence the depth at which objects may be seen. Viewing the light reflected from a submerged white disc through a spectroscope, the red and yellow colors were found to be rapidly absorbed. As it was sunk deeper in the sea a portion of the green became absorbed, the other colors remaining unaltered. The authors remark that this luminous absorption of the more refrangible rays is what would be expected from the calorific opacity and the actinic transparency of water. From the foregoing results, they doubt whether the bottom of the sea has ever been seen at a depth of 100 metres, as it is more probable that the mud and sand brought up by waves has been mistaken for such: the fact that the bottom of the sea is a worse reflector than the white disc, strengthens this supposition.