20 
Remarks on Daniel’s Hygrometer. 
[Jan. 
Postscript. 
Since writing the above, I have met with a paper, by Mr. Adie, m Brewster s Jour- 
nal of Science, No. 1. N. S. on this subject, the results of which are important. 1 
think Mr. Adie deserves great credit for the communication and the experiments 
on which it has been founded; for, to detect and expose the errors of an instrument 
generally confided in is scarcely a less benefit to those who give their tune and at- 
tention to observation, than the invention of a new and perfect instrument, llie 
details brought forward in this paper, form an additional reason too tor o > errors 
to take nothing on trust, but “ to try all things and hold fast by that which i s good. 
In all that I have advanced on the subject of Daniel’s Hygrometer, 1 never ques- 
tioned the accuracy of the instrument, provided it could be made to act. J lie fact 
was, that never having succeeded in obtaining a result from it, on account of the 
badness of my ether, I could not judge whether its indications would be correct or 
not. Mr. Adie at once asserts, that it is not only objectionable in principle, but that it 
actually gives erroneous results; never scarcely showing the true dew point, and in a 
mean of 28 observations, giving a dew point 3° higher than the truth. 
Mr. Adie begins his paper by adverting to Mr. Daniel’s objections to Mr .Tones’ 
simplification of his instrument, briefly noticed in my communication, published 
in the number of theGlcanings forOclober, p. 309, and asserts, that the .same objec- 
tion applies to his own, but in a reverse sense. He observes, “The ether inclosed in 
the bulb on winch the deposition is observed, being cooled by the evaporation from 
its surface, tbe whole mass must acquire the temperature from thee inducting power 
of the fluid alone ; and as the enclosed thermometer is half immersed in tbe ether, 
and half exposed to the temperature of its vapour, while the deposition takes place 
only on a zone at the surface of the ether, a zone only, on the bulb of the enclosed 
thermometer is exposed to the dew point temperature, the other p. rt retaining tbe 
temperature of tbe ether below, and of tbe vapour above; thus the instrument gives 
a dew point always at a higher temperature than the truth.” 
This point, he states, was noticed by Mr. Foggo, of Leith, who in consequence re- 
commended the simplification proposed by Dr. Coldstream of Leith, nearly the same 
as that of My. Jones’. This is the arrangement mentioned in my communication, p. 
309, but attributed to Mr, Foggo. Hut Mr. Adie shows, that tbe objections applica- 
ble to the instrument of Jones applies also to this modification ; as there is no certain- 
ty that tbe whole of the bulb will be of tbe same temperature, when only half of it is 
subjected to the refrigerating power of evaporation, the other half being exposed to 
the atmosphere. In the case ot a globular bulb, the error was 4°, 75 on a mean of °8 
observations ; in the case of a cylindrical one <i The method by which these de- 
ductions were obtained, is that of Le Roy, described in my paper on Hvgrometrv p 
46, erroneously attributed by Mr. Adie to De Saussure. * 1 * ' ° - ’ P ’ 
In conclusion, Mr. Adie proposes the following arrangement, as preferable either to 
mlar case of black glass is pr 
i , with an adjustable collar for 
j ,i , ... , . -. ith alcohol, and tbe outer bulb beimr co- 
ter' ether V, ipplied to Sp0t f° Ut * 9 narter of ■>*> inch in .liame- 
S; r^ducL an lTZi e tt.nu^T ° f 'r'’ ' Vhen 8 “ l-MH-raiurc i" 
mean of 28 observations, the error of 'this instrument^ 0 * 0t V 16 black b,,lb - 0,1 a 
error 5° ; whereas in Mr Daniel’s . ^Lament was only l" and tbe greatest 
frequently 5°. ^ lament, the error was occasionally 7°, and 
very'muct ?\° f " *«* — icaiion. 
rendering it not only cheaper and more portable S 1 J nstrument ? thus » 
have a common thermometer to convert it 1 ’ but also enabling all those who 
And this has been effected, certainly with not 000 "’y a dew point hygrometer. 
It does not appear to me, that the abo^ f- a " y sac . r,face of accuracy. ' Secondly. 
meter, are insuperable ; fir may Adic ’ to Denier* «ygm- 
he requires with his own, be sufficient to ™® a £ ltat,on of Daniel’s instrument, vVn.ch 
remark, even supposing it well founded accurate result. Thu 
suggestion, however, in the view of convenient * ? , , n ? 1 l . msh the y alue of Mr. Ache’s 
it right to notice the subject, as well for tl 1 ‘• ty ’ Rntl econoi "v; but I think 
for the satisfaction of those who may l! ive nnr^l ° ♦ juStice to a11 parties, as 
mg the above suggestion to be weU founded \' S foment, which suppos- 
tinue to be preferred,. though for the tmr, ’ "*1 ft>1 tbe Moratory, perhaps, con- 
ever its accuracy. Thirdly . x should think* ° v tl J a - e, ! er less convenient, wbat- 
ink a cylindrical bulb more likely to gire 
Mr. Daniel’s or Mr. Jones’ instrument. A globular case of Mnnt . -a a 
it° ^ Vh7s the b t Ul t of an ordinary thermometer, with an adjustable^collar flir fixing 
it. The space between the bulb is fitted with alcohol, and tho Alitor Kill li l.otr... 
