4a 
1805. ] 
ing the furface of an oblique cylinder, 
with its geometrical demonftration.”” The 
difficulty of finding the furfaces of an ob- 
lique cylinder, and of an obtique cone, the 
baie of which are circles, is noticed by 
the Jearned Dr. Barrow, in his Geome- 
trice Leéliones. According to our au- 
thor, Mr. Brinkley, ‘* The furface of an 
oblique cylinder is equal to a rectangle 
contained by the diameter of its bafe, and 
the circumierence of an ellipfe ; the axes 
of which are the length and perpendicular 
height of the cylinder.” This theorem is 
demonftrated by pure geometry, accord- 
ing to the method of the ancients, by 
means of circumicribed and infcribed 
prifms. 
To this theorem is added an appendix, 
containing {ome observations on the me- 
thods of finding the circumference of a 
very excentric ellipfe ; including a geo- 
metrical demonftration cf the remarka- 
ble property of elliptic arcs, difcovered by 
Count Fagnani. By the aid of a certain 
property of the elliple, bere given, Mr. 
Brinkley derives the following theorem, 
re{pecting a very excentrie ellipfe. 
‘« If that femi-diameter of an ellipfe be 
taken, which is a mean proportional be- 
tween the femi-axes, and be produced to 
meet the circumicribing circle; then the 
point, where the ordinate to the circle,” 
drawn from the point of interfeQlion, cuts 
the ellipfe, divides the quadranta] are of 
the chipfe into two parts, the difference 
of which is equal to difference of the femi- 
axes.” 
Dr. Ricuarpson has laid before the 
Royal Academy, * An Account of 
Whynn Dykes, in the Neighbourhood of 
the Giant’s Cauleway, Buallycaiile, and 
Belfat.’’ Atter having accurately de- 
cribed thefe dykes, the Doétor fays, 
** Whether cur Whynn Dykes be identi- 
cally the fame with thole on the Scorch 
coatt, oppofite, is not eafily afcertained, 
though highly probable ; but even con- 
fining ourfeives to our own country (Ire- 
land), we find fuficient matter for aito- 
nifhment in contemplating our bataltic 
area, formed by accumulations of hesi- 
zonta) ftrata, with numberlels vertical 
planes radia‘ing from it. Had Dr. Bed- 
does been acquainted with this ftru@ure 
of our bafaltic country, I think he would 
fearcely have afferted that, ‘aright know. 
ledge cf bafaltes is condusting us fait to a 
jult theory of the earth.’ 1 think very 
differently from Dr. Beddoes, and conceive, 
that inttead of affiiting, balaltic facts are 
throwing new difficuities in the way of 
Monraty Mac. Ne. 123. 
Proceedings of learned Societies. 
O21 
cofmogonifts, who flatter themfelves they 
have developed the fecret of nature ; and 
that thofe in my country are utterly irre- 
concileable to any theory that I have met 
with, 
« Two fees of naturalifts, diftinguifhs 
ed by the names Volcanilts and Pluton- 
ifts, have, of late, taken pofleffion of all 
the bafalt world, and have div‘Jed it be-- 
tween themfelves, under the defcription of 
erupted and unerupted lava; and they 
have fo convinced Dr. Beddoes of the va- 
lidity of their claim, that, he fays, I thall 
affume the origin of bafaltes ‘ from fub- 
terranean fufion to be thoroughly efta- 
blifhed.”’ 
According to Mr. Richardfon, foreigners 
feem to know hittle of Whynn Dykes, ex- 
cept ia mines; and, in teis country, Dr.’ 
Hamilton, en behalf of the Volcanilts, and 
Dr. Hution, the great advocate for the Plu- 
tonic fyfiem, have their different theories ; 
the fi: ft forms the Whynn Dykes by pour- 
ing in erup ed fava at the upper aperture 
of mighty chafms, while Dr. Hutton con- 
ceives thele chafms were filled up by his 
own unerupted lava, forced up at the 
lower aperture. Our author, however, 
will not admit that either of thefe gentle- 
men have difcovered the fec et of nature 
in the conftructicn of the fingular appewr- 
ances exhivited in thefe Whynons: for 
rft. Many of our contiguous dykes 
differ materially froos each other, yet their 
proximiiy is fuch, that, according to the 
theory of either Dy. Hamilton or Dr Hut. 
ton, they mult have -been filled up from 
the fame fource, and with the fame ma- 
terial, 2dly. Many of thefe dykes, both 
i) Ireland and Scotiand, thow a material 
difference between their mi‘dle parts and 
thei fides, boih, in gtaio, and internal 
principle of conltru€tion ; a:d the change 
appea s as af the diffiiailar parts were fe- 
parated from each orher by planes paral- ‘ 
Jel to their fides, which 18 in ompetible 
with the high flare of fl -idity in which 
lava mutl have been to enable it to fill up 
valt chotins of fich dimiutive breadth, 
3dly. Whvun Dykes come in contaét 
with a erect variety of different fubltances, 
wihout producing fuch effect upon any 
one of them, as might be expected from 
the contiguiiy of fo glowing amafs. gthly. 
All fubitances, whenionited, are ina high 
{tate of dilatation, which ts followed, when 
they coo!, by a contraétion, by witch they 
occupy lefs Ipace than they did when heated 5 
ot courfe, had the dykes been chaims, 
filled up with glowing lava, when this 
material cooled and contracted, it could 
nv donver fill up thefe cuafms as before, 
ae but 
