i»i:i:i'-sKA Kxn.oitATiox. ].")! 



dccp-sea tenipcratuivs. But tla- instrument .sufni.s to have 

 been forgotten even in England, lor, in tlie subsequent 

 voyages of Beeehcy (1825) and LUtke, (182(;) only common 

 thermometers were supplied, and the device seems to have 

 been inde|)endently re-invented by Glaishcr, Xegretti, and 

 Miller, within the last twenty years. As the thermometei-s 

 most commonly used for this purpose are made by the firm 

 of Casella, in London, they are usually called Casella ther- 

 mometers ; Xegretti and Zambra, of London, have invented 

 another form of thermometer less convenient in size and 

 shape than the Casella. but which offers a meritorious ad- 

 vance in some other directions. 



Metallic thermometers, registering temperatures by the 

 unequal contraction and ex[)ansion of different metals com- 

 bined in one instrument, have been invented, and, while 

 perfect in theory, have not been found satisfactory, except 

 in moderate dej,)ths, when put to actual test. 



Having described the various instruments employed, it 

 remains to indicate briefly the manner in which they are 

 used, premising that, for the minute det-ails of operations of 

 some complexity, it will be necessary for those interested to 

 refer to special treatises on the subject. 



For good work in great depths of water a steam vessel is 

 ab.^olutely necessary. It is important that the lead line, 

 whether of rope or not, should descend pcrpcntlicularly, as 

 otherwise the length of line run out does not indicate the 

 true depth of the water, and greater strain on the line is in- 

 volved in hauling in. Now at sea there is always some 

 wave motion or swell, even in the calmest weather, and un- 

 less the ves.sel can be kept in her original position by a 

 gentle use of the propelling machinery she will certainly 

 drift away from the spot at which the lead was dropped into 

 the water. In running lines of .soundings the u.se of .^^tc^rn 

 is also necessary to continue on the desired course irrei^pect- 

 ive of the winds. The distance between any two staitons, 

 not tixed by astronomical observation or bearings on land, 

 can also be determined more accurately in the case of a ves- 

 sel under steam. A steam-engine is also re(piired to haul 



