24G 



they represent very approximately the circumstances of an attracted 

 point placed at high water-mark on a vertical sea-wall facing a basin 

 or estuary. The radius of the attracting mass of water being repre- 

 sented (more accurately) by 10,000, 20,000, and 40,000 feet, I 

 find the influence of a tide-wave 100 feet thick upon a plumb-line to 

 produce a deviation of only 0-"44 (forty-four hundredths of a second), 

 0'"50, and 0-"53 ; the effect increasing extremely slowly with the 

 radius, as might be expected. If the tide rose only fifty feet, the 

 first effect would be reduced to 0'"246. 



Even the greatest of these calculated deviations affords no ground 

 for hoping that the method of Robison could be applied with any 

 success to determine the earth's density. 



It is rather singular that this ingenious suggestion is not once al- 

 luded to, so far as I am aware, by any writer on the figure and den- 

 sity of the earth; yet surely it was as worthy of notice as Dr Hutton's 

 proposal to measure the attraction of an Egyptian pyi'amid. — (^Phil. 

 Trans. 1821.) 



The following Donations to the Library were announced : — 



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