U 
142 Mr. A. Cayley on the Equipotential Curve ~ +5 =C. 
to believe the former to be of eruptive origin must be prepared 
to extend their incredulity to the latter. Indeed the elevation- 
crater theorists usually do not shrink from this consequence. 
With them the cone of Vesuvius, and that of Monte Nuovo itself, 
were not the products of eruption, but of elevatory expansion by 
a single shock. Obviously, it ought to follow, that no volcanic 
mountain was ever in eruption at all, that the whole is an ocular 
illusion ; at least, that the lava-streams we see pouring for weeks 
and months from the summit of a cone and hardening there, and 
the enormous showers of fragmentary matter which, during 
equally long periods, we see thrown up from the crater and fall- 
ing on the surface of the cone, do not, even in the lapse of ages, 
add to its bulk, or tend by their frequent repetition to compose 
the substance of a volcanic mountain, but by some unaccountable 
process disappear without leaving a trace behind. I own that, 
to my mind, such an hypothesis is wholly unintelligible. I see 
in the ordinary phenomena of a volcanic mountain, such as I 
have described them in the brief record of the principal erup- 
tions of Vesuvius during the last century, a simple and natural 
process by which such a mountain is gradually built up; and 
having observed this mode of formation going on in some in- 
stances before my eyes, I think it reasonable to apply it to 
explain the mode of formation of other mountains of the same 
class with their cones and craters, old and new, central and lateral, 
or parasitic ; and making allowance, as I said at first, for a cer- 
tain amount of internal accretion and elevation, by means of in- 
trusive dykes and earthquake shocks, I know nothing in the 
appearance, figure, or structure of any voleanic mountain yet 
discovered, which such an ordinary and observed mode of forma- 
tion will not account for. 
[To be continued. } 
! 
XIV. Note on the Equipotential Curve < + 5 =(, 
By Artuur Cayuey, Esq.* 
! 
tan 
HE equation 3 + ree, where m, m', C are constants, and 
r, 7! are the distances of a point P of the locus from two 
given points M, M! respectively, expresses that the potential of 
the attracting or repelling masses m, m! has a constant value at 
all points of the locus. The locus is obviously a surface of revo- 
lution, having the line through the points M, M! for its axis; 
and instead of the surface, we may consider the section by a 
plane through the axis, or what is the same thing, we may con- 
* Communicated by the Author. 
