82 Mr. H. F. Talbot's Proposed Philosophical Experiments. 



it in such a position that (the mirror being at rest) the images 

 of the two sparks coincide, or are superposed one upon the 

 other. 



This being effected, let the observation be made through a 

 fixed tube, placing the combined image exactly in the centre 

 of the tube; then if the mirror be made to revolve with 

 great speed, if any separation of the combined spark into two 

 take place, it will be a proof of the existence of an interval of 

 time between them. 



The necessity of the tube is apparent ; for if the eye were 

 directed to other points of the revolving mirror, the two images 

 would appear separate from the mere effect of perspective. 



2. Proposed Method, of ascertaining the greatest Depth of the 



Ocean. 



This is a problem concerning which we can hardly be said 

 to possess any certain knowledge ; since our power of sound- 

 ing does not extend much beyond a thousand fathoms. But 

 in the central parts of the Atlantic and Pacific, the ocean may 

 be many leagues in depth; — at least such is the confident 

 opinion of Laplace. As this is one of the most important 

 points in the physical constitution of the globe, it would be 

 very desirable, if possible, to reduce it to the test of actual 

 experiment. 



The method which I would propose, with some hope that 

 it would prove successful, is to let fall from the deck of a ship 

 one of the newly-invented percussion shells, which would ex- 

 plode on striking the ground ; and the interval of time before 

 the explosion was heard, would give the depth of water with 

 great accuracy. The experiment should be first tried in a 

 known depth of water, say a hundred fathoms, or whatever 

 lesser depth would be consistent with security. The descent 

 of the shell through the water would after the first few se- 

 conds be uniform, as is well known to be the case with all 

 heavy bodies moving in a resisting medium. The time taken 

 by the sound in returning through the water might be neg- 

 lected, unless great accuracy were required ; since it would 

 move at the rate of a mile in half a second. 



If it should be objected that the report of the shell might 

 not be audible at great depths, I would remind the reader that 

 in M. Colladon's experiments the sound of a bell was distinctly 

 heard through the water of the lake of Geneva for a distance 

 of nine miles. 



[To be continued.] 



