EQUIPMENT. 



15 



Sir William Thomson would find it difficult indeed to recog- 

 nize his original machine as now used on board the " Blake," 

 with the modifications introduced by Lieutenant -Commander 

 Sigsbee. During the four years of his command, the latter 

 ran no less than 12,766 nautical miles of sounding-lines, with 

 the necessary serial lides of temperatures. As the result of his 

 magnificent work, the Coast Survey is publishing a hydro- 

 graphic chart of the Gulf of Mexico, not merely unequalled 

 for its accuracy, but unique as the first chart of any extent 

 which carries the littoral hydrography to great depths. 



The depth having been obtained, the thermometer attached 

 to the stray line will, if allowed to remain long enough before it 

 is drawn up, record the bottom tem- 

 perature. The deep-sea thermometer 

 used is known as the Miller-Casella. 

 (Fig. 11.) It is a maximum and mini- 

 mum thermometer. Its modified form 

 was suggested by Professor Miller, 

 and constructed by Casella. It con- 

 sists of a U-shaped tube, the lower 

 part of which is filled with mercury, the U ter- 

 minating on the one side in a large bulb filled 

 with a mixture of creosote and water, while the 

 small bulb of the other branch is filled onlv in 

 part with the same liquid. A steel index, kept 

 in place by a horse-hair spring, is placed in each 

 limb. The indications are given by the expan- 

 sion and contraction of the mixture in the large 

 bulb, which allows the mercury either to recede 

 from the large bulb or to flow towards it ; the 

 index is carried on either side by the movement 

 of the mercury, and on the retreat of the mer- 

 cury the index remains at the highest 

 point reached. To prevent the effects 

 of the enormous pressures to which 

 these thermometers are subject, the 

 inner largfe bulb is protected by an 



o r J Fiff. 11. Miller-Casella Ther- 



outer bulb nearly filled with alcohol, mometer. Half size. 



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