396 Royal Society : — Dr. W. A. Miller on a Self-registering 



the point due to the effect of mere temperature, and the records fur- 

 nished by the instrument rise too high*. 



A simple expedient occurred to me as being likely to remove the 

 difficulty ; 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 

 employ 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 ordi- 

 nary use of the instrument while raising or 

 lowering it into the water. Several of these 

 thermometers have been prepared for the 

 purpose with unusua. care by Mr. Casella, 

 who hasdeterminedthe conditions of strength 

 in the spring and diameter of tube most fa- 

 vourable to accuracy. He has also himself 

 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 

 thermometers from the effects of pressure 

 consisted simply in enclosing the bulb of 

 such a Six's thermometer 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 dis- 

 place part of the air by means of its vapour, 

 and the outer tube and its contents were 

 sealed hermetically. 



In this way, variations in external pres- 

 sure are prevented from affecting the bulb 

 of the thermometer within, whilst changes of 

 temperature in the surrounding medium are 

 speedily transmitted through the thin stra- 

 tum of interposed alcohol. The thermo- 

 meter is protected from external injury 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 sea-water of sp.gr. 1*027, the pressure in descending increases at the 

 rate of 280 lbs. upon the square inch for every 100 fathoms, or exactly one ton 

 for every 800 fathoms. 





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