SURVEYING THE ROUTE. 27 



the compressed air in the other column. In each Column is 

 an index registering the highest point reached by the mercury, 

 that on the lefc showing the maximum cold and that on the 

 right the maximum heat to which the instrument has been 

 exposed. In addition to the usual scale of degrees Mr. 

 Buchanan had a scale of millimetres engraved on the stem. 

 This affords greater ease and accuracy of reading on account 

 of the millimetre divisions being smaller than those of degrees, 

 while the reading is independent of the degree scale, which 

 may be knocked out of position relatively to the thermometer 

 stem. When constructed, the instrument is calibrated in the 

 usual way by immersion in water at different temperatures, 

 say, for instance, at SOdeg., 45deg., 60deg. and 75deg., and 

 these points are etched on the glass stem. A table is then 

 compiled showing the exact relation between millimetres and 

 degrees. Thus, if SOdeg. corresponded to 87mm., and 45deg. 

 to 120mm., every degree would be equivalent to 



120-87 o.o 



= 2 •2mm. 



45-30 



As regards ordinary errors in reading, therefore (such as read- 

 ing 52 for 57, for instance), one scale affords a good means of 

 oheck on the other, but the millimetre scale would of course 

 be the most correct. Supposing, for instance, the reading is 

 33deg. and 74mm. : on referring to a table it is found that 

 74mm. is equal, say, to 33 6deg., which is therefore the correct 

 temperature. One disadvantage in this instrument is that the 

 indices may get out of place, or bubbles of spirit may get into 

 the mercury column if the line receives a jerk ; but in careful 

 hands it is a very reliable instrument for deep-sea work. 

 Should the instrument encounter on its way down or up an 

 intermediate stratum of water colder than that at the bottom, 

 the reading would not give the correct bottom temperature ; 

 but this is an exceedingly unusual occurrence, and the exis- 

 tence of a colder belt can soon be ascertained by a few serial 

 soundings. For lowering in the sea the thermometer is 

 enclosed in a case, as in Fig. 19. 



The subject of thermometers for deep sea work was ably 

 treated in a Paper read by the late Mr. William Lant Carpenter, 

 B.A., B.Sc, before the Society of Telegraph Engineers and 



