216 Remarks upon Mr. Darnell's Work on Hygrometry. [Sept. 



temperature of 32° ; " that is to say, the barometer for the use - 

 of the Royal Society so carefully made, is to have its observed 

 height corrected for temperature, by using the constant denomi- 

 nator 1 1,664 (64-8 x 180) for each degree of Fahrenheit, or by 

 Mr. Rice's table. This number, however, has no connexion with 

 the computation in question, for seeing that Mr. D. is partial to 

 Dulong and Petit, their number 9990 (55*5 x 180) ought to have 

 been Mr. Daniell's choice ; and with regard to the mean dilata- 

 tion of glass, repeated so malapropos, 1). and P. take no mean 

 whatever, asserting its uniformity, and besides do not deduce 

 the number which Mr. D. ought to have used from any such 

 consideration, but on the contrary and conversely, use that num- 

 ber and the apparent dilatation falsely taken, to find that of 

 glass, where they again err. 



At pages 358 — 9 examples are given for the various correc- 

 tions, but all those for temperature are on the same erroneous 

 principle as that engraved on the instrument ; and previously, 

 page 183, there occurs one where -^ seems to be taken as the 

 fraction of dilatation. This example I profess not to understand, 

 unless it be intended to show that we may take any number at 

 random for this correction. 



In taking the liberty of making these remarks on Mr. Daniell's 

 meritorious work, it may be admitted that, though he and Mr. 

 Rice have inadvertently assumed -^.- as the fundamental number 

 on a false principle, still it may not be so far from the truth as it 

 might have been, since the absolute dilatation of mercury is 

 given so variously, that in adopting the right principle, Mr. 

 Daniell might have a number still more erroneous. 



Important as a knowledge of the real amount of the dilatation 

 of mercury would be, in many scientific determinations, in none 

 perhaps is it of such consequence as in this correction for the 

 mountain barometer, respecting which, nevertheless, there is 

 too much cause for regretting that, even in this age of refined 

 experimental knowledge, we are so embarrassed with conflict- 

 ing authorities, as to be forced to entertain more than a suspicion 

 that the real quantity is still unknown. Mr. Dalton calls it -jL ; 

 Dulong and Petit - r ^-- s \ and General Roy -' ¥ ; while philosophers, 

 of equal and undoubted reputation, vouch for the authenticity 

 of almost every intervening denominator, and some for even a 

 greater. 



If such be the actual state of our knowledge with regard to 

 the quantity of the absolute dilatation of mercury, it is difficult 

 to conceive how Mr. Daniell Can so confidently assert, that 

 " this effect has been most minutely appreciated, and its correc- 

 tion applied with the utmost ease and precision." M. Biot 

 fixes this number at •5- r .' TT , which is generally received in France, 

 and by many here ; yet even this celebrated philosopher is 

 wrong, by having made a false conclusion from his own pre- 



