ASTRONOMY. 3 8x 
mountains are various with refpefl to colour, home being 
much darker than others. The molt lofty mountain on the 
furface of our globe is fuppofed to be Chimboraco, which 
is not 20,000 feet in height: but there are many in the 
Moon which are much higher ; that which is diftinguifhed 
by the name of Leibnitz, is not lets than 25,000 feet. This 
elevation will appear more extraordinary, it compared with 
the Moon’s diameter, of which it is ^whereas Chim¬ 
boraco is not above -j-oVf of that ot the Earth: thuscon- 
tidered, the lunar mountains are near five times as high as 
any on our globe. 
The craters of tire Moon are circular, and furrounded 
with an annular bank of hills: they are remarkable for 
their width, many of them being from four to fifteen geo¬ 
graphical miles in diameter: fome are not deeper than the 
level of the Moon’s furface; others are 9000, 12,000, and 
15,000, feet in depth : that of one, which M. Schroeter calls 
liernouilli, is above 18,000 feet. The height of the an¬ 
nular bank is feldom equal to the depth of the crater which 
it fttrrounds; but the quantity of matter in the one appears 
to be in general nearly equal to the capacity of the other. 
The principal mountains and cavities feem to be connefted 
by a feries of others of lefs magnitude; and fometimes 
by hilly Itrata, which, like the radii of a circle, may be 
traced to a common centre ; this is generally either a moun¬ 
tain or crater, though not of the greateft height or depth. 
Thefe hilly ilrata, which, through fmaller telefcopes, ap¬ 
pear - like veins on the Moon’s furface, have often been mif- 
taken for torrents of lava; none of which, M. Schroeter 
fays, he could ever difeover. 
From all the preceding circumftances, this author con¬ 
cludes, that, whatever may have been the caufe of the ine¬ 
qualities of the Moon’s furface, it mud; not only have ope¬ 
rated with great violence, but alfo have met with great 
refinance; which inclines him to think, that the fubftance 
of this planet mull: originally have been very hard and re¬ 
fractory. He is of opinion that thefe mountains and ca¬ 
vities mult have been produced in confequence of fome 
great revolution occafioned by the aftion of a force direct¬ 
ed from the centre towards the furface, and in this refpeCt 
fimilar to that which gave birth to our volcanoes : but he 
obfer.ves, that we have no reafon to fuppofe that it origi¬ 
nated from fire. In fome places, this force has only ele¬ 
vated the furface, and thus formed hills and mountains; 
in others the ground has yielded to its violence, and has 
either been thrown up as a bank round the crater thus 
formed, or elfe, falling into other cavities, has in part fill¬ 
ed them up; after having exerted its greatefi violence in 
thefe mountainous accumulations, it has diff'ufed itfelf in 
various directions, and produced the hilly ftrata which are 
obferved to diverge from them, like the radii of a circle 
from the centre. In fupport of this hypothefis, it is al¬ 
leged, that the largefi craters have the leaf! depth, and 
that in the deepeft there is the mod equal proportion be¬ 
tween the capacity of the crater and the volume of the an¬ 
nular bank around it: but, betides the grand revolution here 
fuppofed, M. Schroeter is of opinion that there have been 
others of later date and lefs extent ; to thefe he afcribes 
the formation of fecondary mountains, which arife either 
from the middle of the craters of the primary, or from 
the centre of a plain furrounded by a circle of hills ; many 
of thefe have alio craters, and, like the primary mountains, 
are connedled by a- feries of cavities and hilly firata, that 
mark the progrefs of the caufe by which they were pro¬ 
duced. The new crater thus difeovered in the fpot He- 
velius, together with other circumftances here enumera¬ 
ted, feem to indicate that the furface of Moon is far from 
heing permanently fettled and quiefeent. The author’s 
obfervations confirm the opinion that the cavities vifible on 
the lunar furface do not contain water: hence he con¬ 
cludes, that there can be no extendve feas and oceans, like 
thofe which cover a great part of the Earth; but he al¬ 
lows that there may be fprings and fmall rivers. 
As the Moon has on her furface mountains and valleys 
in common with the Earth, fome aftronomers have difeo- 
Vol. II, N0.77. 
vered a Till greater fimilarity, viz, that fome of thefe are 
really volcanoes, emitting fire as thofe on the Earth do. 
An appearance of this kind was difeovered fome few years 
ago by Don Ulloa in an eclipfe of the Sun. It was a fmall 
bright fpot like a fiar near the margin of the Moon, and 
which he at that time fuppofed to be a hole or valley with 
the Sun’s light finning through it. Succeeding obferva¬ 
tions, however, have induced aftronomers to attribute 
appearances of this kind to the eruption of volcanic fire; 
and, on April 19, 2787, Dr. Herfchel difeovered three vol¬ 
canoes in the dark part of the Moon; two of them feemed 
to be almoft extinff, but the third fliewed an aflual erup¬ 
tion of fire, or luminous matter, refembling a fmall piece 
of burning charcoal covered by a very thin coat of white 
afites; it had a degree of brightnefs about as ftrong as that 
with which Inch a coal would be feen to glow in faint day¬ 
light. The adjacent parts of the volcanic mountain feem¬ 
ed faintly illuminated by the eruption. A fimilar eruption 
appeared on May 4, 1783. On March 7, 1794, a few mi¬ 
nutes before eight o’clock in the evening, Mr. Wilkins of 
Norwich, an eminent architect, obferved, with the naked 
eye, a very bright fpot upon the dark part of the Moon; 
it was there when he firft looked at the Moon; the whole 
time he favv it, it was a fixed fteady light, except the mo¬ 
ment before it difappeared, when its brightnefs increafed ; 
he conjeflures that he faw it about five minutes. The 
fame phenomenon was obferved by Mr. T. Stretton, in St. 
John’s-fquare, Clerkenwcll, London. Phil. Travf. 1794. 
On April 13, 1793, and on February 5, 1794, M. Piazzi, 
aftronomer-royal at Palermo, obferved a bright fpet upon 
the dark part of the Moon, near Ariftarchus. Several 
other aftronomers have obferved the fame phenomenon. 
See Memoirs de Beilin, for 1778. 
It has been difputed whether the Moon has any atmof- 
phere or not; and the following arguments have been 
urged by thofe who deny it. 1. The Moon, fay they, con- 
ftantly appears with the fame brightnefs when our atmof- 
phere is clear; which could not be the cafe if file were 
furrounded with an atmofphere like ours, fo variable in 
its denfity, and fo often obfeured by clouds find vapours. 
2. In an appulfe of the Moon to a fiar, when file comes fo 
near it that a part of her atmofphere comes between our 
eye and the fiar, refraftion would caufe the latter to feem 
to change its place, fo that the Moon would appear to touch 
it later than by her own motion (lie would do. 3. Some 
philofophers are of opinion, that becaufe there are no feas 
or lakes in the Moon, there is therefore no atmofphere, as 
there is no water to be raifed up in vapours. 
All thefe arguments have been anfwered by other aftro¬ 
nomers in the following manner*: 1 1 is denied that the Moon 
appears always with the fame brightnefs, even when our 
atmofphere appears equally clear. Hevelius x - elates, that 
he has feveral times found in (kies perfectly clear, when 
even ftars of the lixth and feventh magnitude were vifible, 
that at the fame altitude of the Moon with the fame elon¬ 
gation from the Sun, and with the fame telefcope, the 
Moon and her maculae do not appear equally lucid, clear, 
and confpicuous, at all times ; but are much brighter and 
more diftintl: at fome times than atuothers. And hence it 
is inferred, that the caufe of this phenomenon is neither in 
our air, in the tube, in the Moon, nor in the fpeiftator’s 
eye; but mu ft be looked for in fomething exifting about 
the Moon. An additional argument is drawn from the 
different appearances of the Moon in total eclipfes, which 
it is fuppofed are owing to the different conftitutions of the 
lunar atmofphere. 
To the fecond argument Dr. Long replies, that New ton 
has (hewn (Princip. prop. 37, cor.5), that the weight of 
any body upon the Moon is but a third part of what the 
weight of the fame would be upon the Earth: now, the 
expanfion of the air is reciprocally as the weight that com- 
preffes it; therefore the air furrounding the Moon, being 
preffed together by a weight of one-third, or being attrafh-- 
ed towards the centre of the Moon by a force equal only, 
to one-third of that which attracts our air towards the cen- 
S E tre; 
