11G ANNUAL RECORD OF SCIENCE AND INDUSTRY. 



velocity ; but at 10 meters depth the velocity will be the same 

 in 2.39 years. Periodic changes in velocity are propagated 

 very slowly. He shows that two layers moving in opposite 

 directions can be sensibly in contact with each other without 

 material disturbance. The mean motion of the sea as exist- 

 ing 10,000 years ago would to this day be the controlling 

 factor in the present movements of the ocean. 



In the AstronomiscJte Nachrichten, No. 2226, Gylden pub- 

 lishes a first paper on the Rotation of a Solid Body whose Sur- 

 face is covered with a Fluid. lie remarks that to definitely 

 solve this problem we must have given the form of the solid 

 and the quantity of the fluid. In the case of the earth we 

 know not these data, although we can make approximate as- 

 sumptions. After enumerating some of the uncertainties 

 that surround the problem, and indicating the extreme limits 

 of our knowledge and ignorance, Gylden proceeds to develop 

 the mechanical formula in the most general case practicable, 

 assuming the only external force to be the mutual friction 

 of water and earth. 



An interesting paper on the Drifting Power of Tidal Cur- 

 rents appears in the last number which Ave have received of 

 the Royal Irish Academy (January, 1876). The author, G. 

 H. Kinaham, has studied the subject very carefully on a por- 

 tion of the Irish coast, and submits the following conclu- 

 sions: first, the driftage due to the incoming tidal current is 

 during its progress always going on in deep water, and, more 

 or less, in shallow water; second, the driftage due to wind- 

 waves only occurs during gales, and even then is only due to 

 the waves that break on the shores; third, to prevent the ti- 

 dal driftage groins or piers should be erected; and if the pier 

 is to form a harbor, transverse groins should run out from it, 

 to stop the back-wash generated by the pier; for otherwise 

 this back-wash would carry the drift seaward, to be sucked 

 around the pier into the harbor; fourth, as the wind-wave 

 driftage occurs during gales, and then only on the shore-line, 

 it might be prevented from filling up a harbor by placing a 

 breakwater across the direction from which the prevailing 

 storms come. If such a breakwater were fixed, it would prob- 

 ably help to fill up the harbor; but if it be a floating one, it 

 will break the wind-waves in deep water and destroy their 

 drifting powers, while not interfering with their tidal driftage. 



