154 E. L. DeForest on a method of correcting monthly means. 
when we remember how much difference of opinion there has 
been, and indeed still is, _— pet subject, even in the light of 
large deletions of fossils, n hardly with propriety offer 
even a conjecture as to the sete age of the Chinese coal strata. 
0 recapitulate. One species of Podozamites contained in the 
collection is apparently identical with an American Triassic spe- 
eies; the other more resembles a European Jurassic plant. The 
Pterozamites resembles both Triassic and Jurassic species, but is 
identical with neither. The Pecopteris has certainly a remark- 
able likeness to P. Whitbiensis, which occurs both in the Liassic 
and Oolitic floras; and it is not yet certain ap it is not also 
found in the Carolina and Richmond coal basins. 
The Sphenopieris and Hymenophyllites are altogether new, and 
suggest no affinites of value in this connection—while the "Tax 
ttes, Hquisetites, &c., are too obscure to afford us any help. 
Cleveland, Ohio, Sept. 25, 1865. 
ArT. XXITI—A Second Method of correcting Monthly Means for 
the unequal length of the Months ; by Erastus L. DEForEst. 
In the May number of this Journal I gave a system of neh 
equations for finding the mean temperature, rain-fall, &c., 
any mean month in terms of the means for the three nearest ost 
endar months. This was done on the supposition that the curve 
of daily temperatures for any three consecutive months may be 
represented by a portion of a parabola whose equation is 
y=a+be+cr?, 
A similar system of Sec ong may be found by assuming that 
the 
e curve is of the 
peer eit sin (t—e,), 
which may be written 
y=A-+-B cos +C sin 2. 
M1, ,, 5, Z,, and c, retain the same significations which 
they had in my former article, only instead of denoting days, 
let them denote the Cpa ares, the days being — to 
are in the ratio of 3654 days to 360°. By a process of integra- 
tion similar to that followed before, we shall find that 
m= A+ B[sin (2.-++n,)—~sin $n,]——C[eos $n, ~cos(inz-+n dh 
m=A+— B sin 4n., : 
