1824.] X.'s Reply to Mr. Daniell. 349 



tion, have taken the absolute quantity as the foundation of the 

 requisite correction, it is needless here to name individuals who 

 sanction the practice. There is really no imaginable case in 

 which any other fraction can be applied, as it, and it alone, 

 indicates that alteration in the specilic gravity of the mercury, 

 from which solely the necessity arises for the correction in 

 question. 



It would seem that Mr. D. confuses the nature of thermome- 

 tric dilatation, with that which takes place in the barometer ; 

 perhaps he has not duly reflected that the fluid in the one in- 

 strument is isolated, and in quantity definite, whereas the ba- 

 rometric tube being an open vessel, allows the mercury to have 

 free egress and ingress, whether these motions are caused by 

 change of temperature or pressure of the atmosphere. 



Were a column of mercury contained in a tube closed at the 

 lower end, then indeed its expansion would be expressed by 

 the fraction which Mr. D. contends is the proper one for baro- 

 metric correction (gTr:? )> but this would be a thermometer, the 



action in which depends on vicissitudes of temperature alone ; 

 not like that in a barometer, which is modified, or rather mainly 

 produced, by a cause altogether different. 



A case, however, shall be taken which will, I presume, be 

 deemed conclusive. Suppose three barometer tubes standing 

 in a reservoir, and filled alike with mercury, but that one of 

 the tubes expands by heating, that another contracts, and that 

 the third neither expands nor contracts : no one will pretend to 

 say, that if this apparatus be exposed to various temperatures, 

 the columns in all will not rise to precisely the same height ; 

 here, therefore, as in every other case, the expansion or non- 

 expansion of tubes may be utterly disregarded. 



Moreover, as Mr. D. trusts to M. Biot in another matter, 

 (of which below,) he may like to hear his opinion in this, by 

 way of argumentum ad hominem ; M. Biot says, torn. i. p. 86, 

 " Dans cette recherche, il est inutile d'avoir egard a la contrac- 

 tion du tube du barometre. Ce tube il est vrai, se resserre 

 aussi par le refroidissement ; mais sa largeur n'injiue pas sur la 

 hauteur de la colonne du mercure soulevee par l'atmosphere," 

 and in accordance with this decisive explication, he constantly 



uses the fraction of absolute dilatation -rrr;, for correction as 



to temperature of the column. 



There is but one conceivable way in which the expansion of 

 glass would require to be attended to : if the graduation, or 

 inches for measuring the height of the column, were engraved 



on a plate of that material, then — — , (* 3 /|J_ = It-t^-,) or li- 



* 1162 \\/ x 387 1162/ 



neal expansion, becomes the compensating quantity for 180 



