150 SATURDAY LECTURES. 



Several devices more complicated than the ordinary 

 " water bottle " have been employed for the same purpose 

 with success, especially one invented by Captain Wille, of 

 the Norwegian navy, and the Sigsbee water-cup, but all em- 

 body essentially' the same principle. 



Most of you have seen thermometers which register the 

 greatest rise or fall of the fluid in their tubes by means of a 

 little enclosed index like a double-headed pin, and which 

 are known as self-registering. Such an apparatus is evi- 

 dently necessary to obtain a knowledge of the temperatures 

 of the deep sea, since no one can read the thermometers 

 when submerged, and the height of the column when hauled 

 in will necessaril}' represent only the latest temperature 

 near the surface. 



All attempts to use ordinary self-registering thermometers 

 in deep-sea work failed as soon as tried ; not only because 

 the immense pressure of the sea water at great depths com- 

 pressed the bulb of the thermometer, thus forcing the fluid 

 in the tube to a point far above that which would register 

 the actual temperature, but because the pressure was usually 

 sufficient to crush the bulb and tube into fragments at a 

 comparatively moderate depth. At 2,500 fathoms, which is 

 the average depth of the ocean, the pressure is over three 

 tons to the square inch, which is far beyond the endurance 

 of any ordinar}^ thermometer bulb. To overcome this 

 difficulty, sometimes the entire thermometer and sometimes 

 the bulb of the thermometer has been surrounded by a sec- 

 ond bulb nearly filled with alcohol, so that the outer bulb 

 was supported by the fluid and the air bubble by yielding 

 to compression would relieve the interior bulb from its evil 

 effects. This method has proved completely successful and 

 the thermometers now in use for deep-sea work arc entirely 

 of this description. Those with an exposed stem and double 

 bulb are preferred as responding more readily to the tem- 

 perature, though subject to a slight correction for pressure 

 from which the totally enclosed instrument is free. 



Sir John Ross, on his remarkable voyage in 1818, was 

 supplied with thermometers of this sort, and took the first 



