342 —-Remarks on the Coal Formation of Pennsylvania. 
dh.lr 180° 
const. then A.l.or=h.l.A+nh.Lr .*. aioe and v°= Ve (see 
A 
Prin. B. I, prop. 45, ex. 2,) if n=1, p= 180°, and F=; in 
this case ma r/ PE a1, and (19) becomes r=R’(1 —e cos. v)= 
. 
/ 
trex ~ (24), neglecting quantities of the order e? ; it is evident 
by (11), given at p. 73, Vol. XVI, that (24) shows the curve de- 
scribed, in this case, to be an ellipse, the centre of force being at 
the focus, R’/=the semi-axis=the semi-parameter, neglecting quan- 
tities of the order e? ; also in this case (20), (21) become = 
nt+2e sin. nt, r= R/(1 —e cos. nt) which agree with (s), given at p- 
73, Vol. XX, and (1), given at p. 291 of the same Vol., by neglect- 
ing quantities involving e?, e*, etc. in those formule. 
Art. XV.—Remarks on the Coal Region between Cumberland and 
Pittsburgh, and on the Topography, Scenery, &c- of that portion 
of the Alleghany Mountains ; in a letter, dated Pittsburgh, Nov. 
30, 1831, from Samven Wytiys Pomeroy, Esqr.; written leet 
quest of the Enrror. 
Dear Sir—The advanced stage of the season will deprive me of 
the pleasure of furnishing so promptly as I could wish, in accordance 
with your request, an account of the statistics, as well as facts relating 
to the geology and mineralogy of the coal fields on the panks of the 
Ohio. The detention here has exceeded our anticipations @ fort- 
night. I cannot, however, relinquish the hope that winter will per- 
mit me to stop a few days on our way to Cincinnati, where we shall 
remain while the navigation is obstructed by the ice, which seldom 
continues more than six weeks, when from the knowledge obtained 
by explorations, that occupied three or four months last year, J con- 
fidently expect to be able to put you in possession of the desired in- 
formation by the Ist of March, so far as I may be competent [0 oa 
pleasing task. 
I am gratified, however, that a fortuitous circumstance has enabled 
me to present an imperfect sketch or outline of the coal formation of 
