16 | Reply to the 
found, that when the tar has been nearly evaporated; this yel- 
low farina will begin to pass off. 
It remains for me to propose a name for the white concrete 
substance which has been described in this paper: and, unless a 
more appropriate term should be suggested by others, I would 
propose to call it Naphthaline. 
III. Reply to the “ Apology for the. Postscript on the Refrac- 
tions”’ in No. 24 of The Quarterly Journal of Science. By 
James Ivory, M.A. F.R.S. 
To Dr. Tilloch. 
Sir, — I HAVE to request the favour of your inserting the fol- 
lowing observations in reply to an article that has appeared in 
the last Quarterly Journal of Science. I shall take no notice of 
what is merely personal; but it would not be right to allow a 
writing so entirely calculated to mislead, to go before the public 
without making some attempt to enable it to judge of the merits 
of the case. 
Although drawn up with some art and great apparent confi- 
dence, the article, in fact, leaves the observations I wrote on 
the new method of computing the refractions just in the same 
predicament they would be, if no such apology had been pub- 
lished. 
I found that the series, or the development of the density of 
the air in terms of the refraction, was not sufficiently convergent 
to he of use. Does the author contradict this? He does not: 
on the contrary he allows it, by flying off to a different and more 
laborious method of computation, which has nothing to do with 
the construction of the table in the Nautical Almanack, the only 
point I proposed to examine, and the only point about which it 
is worth while to bestow a thought. 
The method he employs consists in considering the variable 
quantities in the several stages of their increase, and computing 
their successive values by repeated operations. It is a method 
resorted to when all others fail. Recourse is had to it here from 
the want of convergency of the series first contemplated, and by 
which his table is constructed, with the hope, no doubt, of re- 
scuing his mode of calculation from the reproach of a total 
failure. 
The methods of calculation proposed by Dr. Young are not 
new, although he may be the only mathematician that has ap- 
plied them to the problem of the refractions. They are the first 
that occurred in the progress of the integral Calculus. Would it 
not 
