VISION UNDER WATER. 39 



different situations, has yaried as far as from 6 to 15 tons Considerations 

 to the height of 100 feet. But although the rate at respecting me^ 

 which mechanic force is generated may vary, any quan- chanic force. 

 tity of work executed is the same, in whatever time it 

 may have been performed. 



In short, whether we are considering the sources of 

 extended exertion or of accumulated energy, whether 

 we compare the accumulated forces themselves by their 

 gradual or by their sudden effects, the idea of mechanic 

 force in practice is always the same, and is proportional 

 to the space through which any moving force is exerted or 

 overcome, or to the square of the velocity of a body in 

 which such force is accumulated. 



V. 



Letter from a Correspondent^ affirming^ contrary to some 

 Observations in our last Number^ that objects can be 

 distinguished by the Human Eye under IVater^ with ad- 

 ditional experiments by the Editor, 



London Institution^ 22d Aug. 1806. 

 To Mr. Nicholson. 

 Sir, 

 I have just laid down your Journal for the present 

 month, and feel myself not a little surprised at your dis- 

 sertation on swimming, and your observations on what 

 Dr. Franklin has written on that subject. 



Your objection to the Doctor's mode of giving confi- Remark on Dr 

 dcncc to those who wish to acquire the art, appears to Franklin's «jp- 

 mo not founded on fact, at least not general fact; you ^b"^\°^i ^^1^. 

 seem to think the Doctor's plan like the senatus consul- distinguished 

 tum of the mice, a ve^y good one, but quite impracti- ^y <3i^s^^* 

 cable : now I find two very learned men differing upon a 

 particular ascertainable point, ascertainable to any one 

 who has eyes formed without some very uncommon de- 

 fect, and who has, at the same time, courage enough to 

 plunge his head under clear water. (For the sharpest 

 sighted man cannot see in water, where the light is shut 

 out by mud floating in it, or by the sides of a vessel that 

 is too small to admit its rays, any more than in a room 

 where light is shut out by the window shutters being 

 closed). 



2 I have 



