96 Mr. E. C. Summers on a Simple Apparatus 



I have calculated the horizontal attraction of a semicylinder 

 of water 100 feet thick, and of about two, four, and eight miles 

 radius upon a point at the extremity of the axis of such a se- 

 micylinder; because these conditions can easily be reduced to 

 calculation, and because 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 represented (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 ap- 

 plied with any success to determine the earth's density. 



It is rather singular that this ingenious suggestion is not 

 once alluded to, so far as I am aware, by any writer on the 

 figure and density of the earth ; yet surely it was as worthy 

 of notice as Dr. Hutton's proposal to measure the attraction 

 of an Egyptian pyramid. (Phil. Trans. 1821.) 



XIII. On a Simple Apparatus for Washing Precipitates. 

 By Eustace C. Summers*. 



MY attention has been lately drawn to an account by M. 

 Bloch, given in the May Number of the Annales de 

 Chimie et de Physique^ vol. xxvi. p. 126, 3rd series, of a new 

 method of washing precipitates by means of a self-regulating 

 siphon. The instrument proposed by M. Bloch is, however, 

 open to several serious objections. In the first place, as soon 

 as the requisite quantity of water has been supplied to the 

 filter, the water rises in" the exterior air-tube to the level of 

 the liquid in the vessel from which the supply is obtained. 

 Now if the precipitate be very light, part of// also is liable to 

 gain access to the tube and adhere to its sides. Again, when 

 the level of the water in the filter falls, the column of water 

 in the air-tube does not always fall so as to leave free access 

 to the air. On the contrary, the whole or part of it is fre- 

 quendy carried upwards by the pressure of the air, and falls, 

 not into the filter, but into the reservoir; and should any of 

 the precipitate be carried with it, which is far from improbable, 

 of course an analysis might be at once invalidated. I say 



* Communicated by the Author. 



