298 THE DEPTHS OF THE SEA. [CHAP. VII. 
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 :— 
| 
| ERROR. | Per 250 FaTHOMS, 
Pressure. | 
| Press. Ocean. | Press. | Ocean. 
Fathoms. A s: | 5 
250 0 7260, | O° 738.0. |) 0° 7266.4) Osa 
500 1 + 548 15564 I Or aii | O =4782 | 
750 2-123 2°223 | 0° 708 | (0.°745leeaee 
| 1,000 2° 474 3° 015 | 0° 674 0: 754 
| 1,250 Br25o | 9B 492 || 0 6b: 0 - 698 
| 1,500 4:107 fe eeg ail | O° 684 0 + 653 
1,750 4 +555 4-056 0 + 650 LO ra 7/09) 
2,000 De oo 4 4° 284 0 - 669 0: 536 
2,250 6: 021 — 0 + 669 a 
2.500 6° 817 at 0 =682: | wae 
For taking bottom temperatures at great depths 
two or more of the Miller-Casella thermometers are 
