344 J.D, Dana on the Volcanoes of the Moon. 
nite extent; for the size of a boiling lake can have no limits ex- 
cept such as may arise from a deficiency of heat. The size of 
the lunar craters is therefore no mystery. Neither is their cireu- 
lar form of difficult explanation; for a boiling pool necessarily, 
by its own action, extends itself circularly around its centre.* 
The combination of many circles, and the wail sea-like areas 
are as readily understood.+ 
. With so perfect. a correspondence, and so satisfactory pane 
nations by means of an appeal to facts, it is hardly necessary to 
enter a protest against the ordinary view that these craters are 
the result of cinder eruptions.{ We remark only that such erup- 
tions will never take place except from small vents, for the cold 
which gives the viscidity on which they depend, necessarily 
contracts the area. Ina large pool the fluidity is such that the 
rising vapors pass off freely: the ejections over its surface, ex- 
cepting those at the margin, will: fall back again into the pool, 
as in boiling water and Kilauea, and could neither rise to the 
height nor make such curves as are represented by Mr. Nasmyth. 
Instead of a large open crater having greater projectile force im 
proportion to its size, it will actually have far less; and within 
certain limits the force may be inversely as the diameter, shone 
dependent also on the size of the chimney above. 
Any vents in the moon in which the fires had etnies aie 
sided, would have densely viscid lavas from partial cooling ; and 
in these there would be loftier ejections like those of ordinary 
volcanoes, forming high conical peaks with narrow —_ if 
any, at summit. * 
The great depth of the lunar site seems to require aie ale 
ment for its explanation, in addition to what has been presented. 
This is supplied by the fact of the less specific gravity of sub- 
stances on the moon; for objects on its surface have but one-sixth 
the ene gravity diay have on the earth: that is, iron would 
aoe 
* Mr. Nasmyth suggests that the quietness of the lunar meet may account 
for the regularity of the circles. 
t M. Rozet observes, in his article referred to, that the moon’s craters do not 
resemble oe riseoe volea noes: and he explains them by supposing that during 
the cooling of the moon’ cireular flowings, which 
carried the scoria o— the centre to the sisnameenene, and thus accumulated the - 
ng ridges. We see no cause for gd existence of ba aed nor _— ese! 7 
It from move! ’ 
mente 
¢ See the i d rk of Mr i taanateien 
a Baar epee ah uit ei 
