298 



THE DEPTHS OF THE SEA. 



[CHAP. vn. 



This, therefore, may be regarded as a perfect instru- 

 ment for all ordinary purposes. 



A number of the instruments which had been 

 previously tested in the press were sent out in the 

 ' Porcupine ' on her summer cruise in 1869, and on 

 her return the results of Captain Calver's observa- 

 tions at different depths in the ocean were carefully 

 compared with the effects of equivalent pressures ap- 

 plied to the thermometers in Mr. Casella's ( Bramah's 

 press.' The result in the ocean, contrary to that in 

 the hydraulic press, proves that the elasticity is not 

 regular or in a ratio to the pressure, but that after 

 continuing regular up to a pressure of 1,000 fathoms, 

 it decreases in a compound ratio to a pressure of 

 2.000 fathoms, when its elasticity nearly ceases. 



The following table gives an abstract of the 

 behaviour of Casella's ordinary Hydrographic Office 

 thermometers in the ocean and in the press : 



For taking bottom temperatures at great depths 

 two or more of the Miller-Casella thermometers are 



