1869.] 



adapted to Deep-sea Soundings. 



483 



culty ; and as upon trial it was found to be perfectly successful, I have 

 thought that a notice of the plan pursued might not be unacceptable to 

 future observers. 



The form of self-registering thermometer which it was decided to em- 

 ploy is one constructed upon Six's plan. Much care is requisite in 

 adjusting the strength of index-spring, and the size of the pin, so as to 

 allow it to move with sufficient freedom when pressed by the mercury, 

 without running any risk of displacement in the ordinary use of the in- 

 strument while raising or lowering it into the 

 water. Several of these thermometers have been 

 prepared for the purpose with unusual care by 

 Mr. Casella, who has determined the conditions 

 of strength in the spring and diameter of tube 

 most favourable to accuracy. He has also him- 

 self had an hydraulic press constructed expressly 

 with the view of testing these instruments. By 

 means of this press the experiments hereafter to 

 be described were made. 



The expedient adopted for protecting the ther- 

 mometers from the effects of pressure consisted 

 simply in enclosing the bulb of such a Six's ther- 

 mometer in a second or outer glass tube, which 

 was fused upon the stem of the instrument in the 

 manner shown in the accompanying figure. This 

 outer tube was nearly filled with alcohol, leaving a 

 little space to allow of variation in bulk due to 

 expansion. The spirit was heated to displace 

 part of the air by means of its vapour, and the 

 outer tube and its contents were sealed hermeti- 

 cally. 



In this way, variations in external pressure are 

 prevented from affecting the bulb of the thermo- 

 meter within, whilst changes of temperature in the 

 surrounding medium are speedily transmitted 

 through the thin stratum of interposed alcohol. 

 The thermometer is protected from external in- 

 jury by enclosing it in a suitably constructed 

 copper case, open at top and bottom, for the free 

 passage of the water. 



In order to test the efficacy of this plan, the instruments to be tried 

 were enclosed in a strong wrought-iron cylinder filled with water, and 

 submitted to hydraulic pressure, which could be raised gradually till it 

 reached three tons upon the square inch, and the amount of pressure 

 could be read as the experiment proceeded upon a gauge attached to the 

 apparatus. 



Some preliminary trials made upon the 5th of May showed that the 



