1824.] Remarks upon Mr. Daniell's Work on Hygrometry. 217 



raises, Traite, tome i, page 52. From what he there states, a 

 different number ought to have been obtained ; and the error 

 affects many of his subsequent formulae relating to the barome- 

 ter, expansion of fluids, of gases, &c. 



In common with many others, Mr. D. seems to think that 

 boiling the mercury in the tube of a barometer is of great con- 

 sequence ; this is at least doubtful, but certain it is that no 

 human art can render a mass of this fluid such mere mercury 

 that it shall not contain something, which in all its mechanical 

 effects may not be called air. Sir H. Davy is in the right when 

 he says so ; and it is not a little to his credit to have perhaps 

 by induction inferred that such is the fact, when there is no 

 reason to think him aware of certain proofs to which it were 

 needless here to appeal. 



Mr. Daniell speaks also of filtering the mercury ; though per- 

 formed a thousand times no good effect can follow this practice. 

 A knowledge of this metal gained from a peculiar application of 

 it, warrants the assertion, that the mercury of commerce is not 

 improvable by either distillation or Alteration, in so far as its 

 application is purely mechanical, and that its fitness for baro- 

 meters can be completely known by bare inspection. 



There is a probable source of error in the barometer hitherto 

 little attended to, and of which Mr. D. takes no notice ; in 

 making the correction for temperature, it has ever been taken 

 for granted, that the expansion proceeds pari passu, or that the 

 fraction of dilatation is, for example, the same from — 20 to 

 — 30, as it is from 90 to 100. This is, however, quite a gra- 

 tuitous assumption ; and although there are reasons for here 

 suspecting something different from the utmost degree of pre- 

 cision, it is possible that the experimental means which we at 

 present possess are inadequate to ascertain any appreciable 

 discrepancies. 



Much learning has been brought to bear on the other cor- 

 rections requisite in using the mountain barometer; and Mr. 

 Daniell could not have conferred a more substantial benefit than 

 by having set the question at rest, as to the absolute dilatation 

 of mercury, without a certain knowledge of which, all other 

 minute attentions are little better than mere drivelling. 



Mr. Daniell's account of the manufacture of barometers and 

 thermometers is most certainly not overcharged. Throughout 

 the continent, and even in England, the business is in the 

 hands of itinerant Piedmontese; and these artists supply not 

 only the general public with their glittering baubles, but furnish 

 the greater part of the most reputable instrument-makers with 

 their whole stock of meteorological wares. Such of these as 

 choose to graduate their own scales, must confide entirely as 

 to the quality of their tubes and the excellence of the filling, 

 in one who has but indirect interest in the matter, or equivocal 



