B6 Mr Maccullagh on the Intensity of' Light. 



kept up upon the bottom of the harbour will prevent the sand 

 being deposited. 



There is another mode already hinted at of sand being collect- 

 ed, which the peculiar construction of harbours here described 

 is equally well calculated to counteract. It is a fact well known 

 to some of those who attentively observe the operations of 

 nature, that all floating substances, especially such as do not 

 rise above the surface of the water, approach the shore with a 

 land wind, and recede from it by a contrary wind. This seems 

 to arise from the violence of the wind upon the water causing 

 the surface water to pass from the shore, and the under water to 

 approach it, to supply its place by means of an opposite current. 

 This under current is the more conspicuous in its operation, 

 when the tide flows or ebbs in a contrary direction to that of 

 the wind, and the floating substances carried along are according- 

 ly deposited. 



The consequence is, that, wherever large solid buildings are 

 erected in a current of water, which has a tendency to sand up, 

 and where these buildings pass along the shore, the wind from 

 the shore that passes over them causes a sand bank to be formed 

 seaward of the building, and to bring it directly into the 

 mouth of the harbour, if it happens to be in the line of that 

 building. This fact is strongly exemplified by the sand already 

 accumulated at the back of the wet docks at Leith, or the stone 

 pier at Newhaven, and in many other situations. 



9, Dean Street, Edin. 

 30th Nov. 1830. 



Art. IX. — On the Intensity of Light when the vibrations arc 

 elliptical. By James Maccullagh, Esq. Communicated 

 by the Author. 



According to the opinions commonly received, the intensity 

 of light, in the undulatory hypothesis, is proportional to the 

 vis viva, which again is proportional to the square of the 

 greatest velocity. Now the greatest velocities will be the same 

 in an ellipse and a right line which have the same period, if the 

 greater axis of the former be equal to the whole extent of the 



