48 
On Hygrometry. 
tion*, will l»e evi ’ent to every one who has considered the subject. It may be 
here sufficient toolierve, that the assumption on which the whole investigation is 
based, is notoriously contrary to experiment Add to this, that the varying specific 
heat of the w.iter for different temperatures must be considered The passage in 
italics is not very intelligible. Why shou d it be necessary to modify the co-effici- 
ent which, it is said, is to be determined by experiment, when that very diminished 
temperature is obviously one of the conditions of the exneriment ? 
In § 5(>, he goes on to say : u It must be evident, however, from the view we have 
taken of the cooling process, that a thermometer with a moistened bulb ought to be 
reduced through the same number of degrees in equal times ’’ Now the very 
reverse of this is evident, if we are o adopt the account of the process given in § 55, 
in which it is stated, that the evaporation being always equal in equal times, depend- 
ing as he says on the constant quantity ( F — j ) is modified hy the influx of caloric, 
which being ne .rly proportional to the depression, increases as that increases, so that 
the apparent velocity of the cooling process is continually diminishing, till at length 
the ingress of heat balances the egress, and the thermometer is then stationary. 
Nothing can he more glaring than this misstatement of his own views, or more cal- 
culated to puzzle the student. 
But these inconsistencies are trifling, compared with the algebraical process em- 
ployed in the treatment of the equation D= m (F — '.) In Art. 57, it is said: tu Since 
the extent of the evaporation, together with the reduction of temperature which it 
occasions, is diminished by the eooli tg of the evaporating surface, the expression 
^ = m ( ^ require some correction, and as this correction must have a 
direet^rolation to 1), the simplest way of applying it is to give the equation this form 
^ — — = - m —J* ) ’ Now it is quite evident, that these two equations are per. 
n 
fectly similar and of the same import, the new form merely leading to a change in the 
value of the constant m, the one being to the other in the ratio of D : D . — 
Indeed, the author himself gives the following transformation of it (F /) 
m n n — 1 
and immediately after, substituting p for it becomes (F —f). This for- 
n — 1 
mula differs in no respect from the original one ; so that the reader is left to wonder 
what correction has been introduced, or how the substitution of p as a constant for 
m can have improved the formula. 
The next step is,however, more important. “ To render the expression consistent 
with the properties of the curve, whose ordinates represent the progressive reduc- 
tions of temperature by evaporation from the moistened bulbs, it teems necessary 
to give it the form D=»f> XI''—//’ &c. Now, it is worthy of remark, that this 
T 
curve, the prnpert.es of which are made to effect this verv important change, is not 
a curve drawn on a scale to represent the actual depressions in equal times nor 
yet one the nature of whtch ,s dehned hy any equation derived from the conditions 
ot the expen, cent, but a mere hypothetical diagram drawn for illustration and as 
X^er!* y ^ ’ Uay COnUeiVed l " Wt Lhu properties of a^’y curve 
The expression having now taken this form l.v a • 
we are naturally led to ask. Would it not have been more^ntellfoJhfo as3um P tlon ’ 
rect to have begun with it as an avowed assumption ? In 
meant to he deduced empirically, the wonder iswhy the aut hor Vh m a 
time and the patience of his readers, in an attempt to obtain >> t, ' ' , waste h ! 3 
on. A more important fault, however, is thaTtU formula tb/ » Pf 
moulded to the form desired, will not represent the fact “ ’ gh so arbitrarily 
to determine the yalue^the^ factors*^ and* * r ’ •» W ^* tate 1’ ‘V was "“dertaken 
• M- Gay- Utssac, whose experiments on the suhiect I L ^ ^ 
appears to have considered the question to be ofsnlnm.i- 6 already referred to, 
be worth the trouble and time which its investigation wm !ill^ ateti a nat ure, as not to 
abiding by the experiment of JLe Roy. In this oninio ) c demand. He preferred 
editor of the Journal of Science. ^ 1011 18 countenanced by the 
