Simple Syphon as a Hydrmneter. 367 



opaque liquid with that of a transparent one, as for instance 

 water.* 



Let a be the volume which the air included in the syphon had 

 under the external barometric pressure 6, and c the increase of 

 pressure occasioned by the immersion which will be proportional 

 to c\ the volume of air below the surface of the water; also let 

 e be the volume of water in the bottom of the tube. Then the 



reduced volume of air is -; , and the volume of the other 



o -f- c 



hquid within the tube, is a 7— — — e = -r-— — — e, which 



subtracted from g^, the whole contents of the tube under the 



CL C 



level of the opaque liquid, gives g-\- e , for the column 



o ~j~ c 

 of air below the surface of the opaque liquid, and if by this we 

 divide c', the quotient is the specific gravity sought. 



In this case, it is supposed that the bore of the tube is uni- 

 form, and that both legs enter their respective liquids at the 

 same instant. But when the two specific gravities are very dif- 

 ferent, unless care be taken not to immerse the syphon too far, 

 some air may escape from the end which descends into the 

 lighter fluid. This may also be avoided, by gradually lowering 

 the vessel containing the heavier liquor, or raising the other 

 whilst the syphon is descending ; but that will seldom be neces- 

 sary, and the escape of a little air will not affect the simpler me- 

 thod, Lo be used when both liquids are transparent. 



On the Live Marine Cockles, said to have been found at a great 

 distance from the Sea in Yorkshire. In a letter to Professor 

 Jameson. By W. C. Trevelyan, Esq. M. W. S., &c. 



1^ Nearly a year ago, my intelligent friend Mr Witham of Larting- 

 ton, had sent to him in Edinburgh several specimens of live 

 marine cockles, said to have been found in a bog, consider- 

 ably above the level of the sea, and fully forty miles distant 



* In some opaque liquids, as for example mercury, the top of the cohunn 

 within the tube may be rendered visible by bringing the leg of the syphon 

 close to the side of the jar ; in such cases, no additional calculation ig re- 

 quired. 



