308 



Royal Society : — 



O 



thermometer could be constructed which could not possibly be put- 

 out of order in travelling or by incautious handling, and which 

 should be above suspicion and perfectly trustworthy in its indica- 

 tions. This was no very easy task. But the 

 instrument now submitted to the Fellows of the 

 Royal Society seems to us to fulfil the above 

 onerous conditions, being constructed on a plan 

 different from that of any other self-register- 

 ing thermometers, and containing as it does 

 nothing but mercury, neither alcohol, air, nor 

 indices. Its construction is most novel, and 

 may be said to overthrow our previous ideas of 

 handling delicate instruments, inasmuch as its 

 indications are only given by upsetting the in- 

 strument. Having said this much, it will not 

 be very difficult to guess the action of the ther- 

 mometer ; for it is by upsetting or throwing out 

 the mercury from the indicating column into a 

 reservoir at a particular moment and in a par- 

 ticular spot that we obtain a correct reading of 

 the temperature at that moment and in that 

 spot. First of all it must be observed that this 

 instrument has a protected bulb, in order to 

 resist pressure. . This protected bulb is on the 

 principle devised by us some sixteen years since, 

 when we supplied a considerable number of ther- 

 mometers thus protected to the Meteorological 

 Department of the Board of Trade ; and they 

 are described by the late Admiral FitzRoy in 

 the first Number of the ' Meteorological Papers,' 

 page 55, published July 5th, 1857. Referring 

 to the erroneous readings of all thermometers, 

 consequent on their delicate bulbs being com- 

 pressed by the great pressure of the ocean, he 

 says: — "With a view to obviate this failing, 

 Messrs. Negretti and Zambra undertook to make 

 a case for the weak bulbs, which should trans- 

 mit temperature, but resist pressure. Accord- 

 ingly a tube of thick glass is sealed outside the 

 delicate bulb, between which and the casing is a 

 space all round, which is nearly filled with mer- 

 cury. The small space not so filled is a vacuum, 

 into which the mercury can be expanded, or 

 forced by heat or mechanical compression, with- 

 out doing injury to or even compressing the 

 inner or much more delicate bulb." 



The thermometers now in use in the ' Chal- 

 lenger' Expedition are on this principle, the only 

 difference being that the protecting chamber has 



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