290 THE DEPTHS OF THE SEA. [CHAP. VII. 
stant in its action, as the amount to which the 
bulb is compressed depends upon its form and upon 
the thickness and quality of the glass; thus the 
error of good thermometers of the Hydrographic 
Office pattern varies from 7°C. to 10°5 C. at a pres- 
sure of 6°817 lbs. on the square inch, representing 
a depth of 2,500 fathoms. In thoroughly well- 
constructed thermometers, however, such as those 
made by Casella and Pastorelli for the English 
Admiralty, the pressure error is tolerably constant ; 
and Captain Davis, R.N., who has lately conducted 
important experiments on this point, expresses his 
opinion that by an extended series of observations 
a scale might be obtained to correct the ther- 
mometers hitherto in use to a close approximation 
to the truth, and thus utilize to some extent obser- 
vations which have been already made with our 
ordinary instruments. 
In the ‘Lightning’ expedition in 1868 we used 
the ordinary Hydrographic Office pattern, and a 
large number by different makers were sent with 
us for testing and comparison. The depths not 
being very great, the general temperature results 
came out well, and were among the most singular 
phenomena which we had torecord. Many of the in- 
struments were very wild at a few hundred fathoms, 
and several gave way under the pressure. On our 
return in April 1869, Dr. W. A. Muller, VoPsnoss 
attended a meeting of the Deep-Sea Committee of 
the Royal Society at the Hydrographic Office, 
and proposed encasing the full bulb in an outer 
covering of glass containing air, in order to permit 
the air to be compressed by the pressure of the 
