/>
to the absence of an atmosphere; and, seen contrasted with the brilliant

lighting of the parts exposed to the full glare of the sun, appeared

almost like deep holes in the lunar surface.



John now remarked, "Professor, you are aware that I have only a rather

vague general knowledge of astronomy, although I take an interest in the

subject, and that I know still less about the dimensions and physical

character of the moon and planets; so perhaps you will be good enough to

give us a little detailed information respecting this beautiful orb.

Most of it will be news to me, and probably it will all be fresh to

M'Allister."



"Heh mon," the latter replied, "just put me among machinery and I'll

tell you what's what, but I never learned anything about astronomy, so

will not pretend to any knowledge of it, but now I should be very glad

to hear what the Professor has to say about it."



"Well, friends," I replied, "it is not my wish to lecture you upon the

subject, so I will merely just run over a few of the bare facts.



"To begin with-the moon is very much smaller than the earth, its

diameter being only 2160 miles, while the earth's diameter is 7918

miles. Being a smaller globe its mass is much more loosely compacted

than that of the earth, so, although it would take nearly fifty globes

the same as the moon to make one globe as large as the earth, it would

require nearly eighty such globes to make one as heavy as the earth.



"The moon's distance from the earth is generally given as being about

238,000 miles, but this is its mean distance. When farthest away from

our world its distance is about 260,000 miles, but at its nearest it is

less than 220,000 miles distant. This difference of course arises from

the eccentricity of the moon's orbit, and it explains why we sometimes

see the moon a trifle larger than it appears at other times. By this I

mean that it really is seen larger, because it is closer to us. But you

have no doubt often noticed that when the moon is near the horizon it

seems to be very large indeed. This apparent increase of size is,

however, an illusion, owing to our unconsciously comparing it with the

apparent size of terrestrial objects.



"The surface of the moon shows evidence of very violent volcanic action

having occurred in every part of it, and astronomers in the past were

much puzzled to account for the excessive volcanic energy which was

indicated by what they saw, as such a small globe as the moon would not,

in the ordinary course of events, have ever possessed sufficient heat to

have developed such violent action. A theory of later years has,

however, provided a reasonable explanation. It is that the moon was at

one time a part of the same mass as the earth, which became separated

from it before the earth had quite cooled down and solidified into its

present form, and was then gradually driven farther and farther away

from the earth by natural forces. It was therefore originally as hot as

the rest of the mass which formed the earth, but being formed into a

smaller globe of much less gravity-only one-sixth of that of the

earth-volcanic action of the same intensity as that on the earth would

have a much more far-reaching effect. A force which on the earth would

project volcanic lava and scoriae a distance of three miles would, on the

moon, project it a distance of eighteen miles. This accounts for the

very high mountains we see on the moon, some of which are comparatively,

for the size of the globe, much higher than those on the earth. It also

accounts for the vast size of the lunar craters, ring-plains, and

ring-mountains.



"These latter are formations quite unknown upon our earth, but on the

moon they are numbered by hundreds of all sizes, from a few miles up to

one hundred and fifty miles in diameter. They are large plains, roughly

circular in shape, and surrounded by mountains; in a few cases the ring

is in some parts a double range of mountains. Sometimes the plain (or

'floor,' as it is termed) is many thousands of feet below the general

level of the lunar surface; in a few cases it is raised considerably

above it, and in one or two instances, instead of being flat, the floor

is convex. Some of the mountain rings are comparatively low, but in

other cases the mountains are fifteen to twenty thousand feet in height,

or even higher. Frequently a mountain rises near the centre of the

floor, some rings containing more than one such mountain, whilst others

have none at all.






IDEAL VIEW OF LUNAR SCENERY



As there is no atmosphere on the moon, the sky is a dense black, and the

stars shine brilliantly in the daytime. The view is a typical one,

showing numerous craters and cracks, and a small ring-mountain with

terracing. Ring-mountains and plains vary from a few miles to 150 miles

diameter, some mountains being nearly 20,000 feet in height.]



"There are numerous instances where one mountain ring has overlapped or

cut into another, thus indicating that it was a later formation; and in

many cases the mountains are 'terraced,'[5] as it is termed, either

owing to a series of landslips or to the rise and fall of a sea of lava,

which cooled as it sank down, thus forming terraces. Small craters

abound all over the surface of the moon and on the floors of the rings;

cracks in the lunar surface are also numerous.



"As regards the lunar mountains, it may truly be said that we have a

fairly accurate knowledge of peaks and mountains which would either be

too precipitous to be climbed, or quite inaccessible to us, if we could

actually land upon the moon; and the whole visible surface has been more

carefully and thoroughly mapped out and studied than is the case with

many parts of our own earth.



"If the moon has any atmosphere it must be so very attenuated indeed

that human beings could not possibly live in it at all; but nothing has

yet been detected which would enable us to say positively that any

atmosphere does exist there, although there have been some indications

observed which support the supposition that there may be an extremely

thin air.



"Nor does it appear possible that there is any water upon its surface at

the present; in fact, many astronomers are of opinion that the moon

never did have any water upon it. Personally, from a study of many of

the formations as seen through the telescope, it seems to me quite

impossible that they could owe their existence in their present state to

anything but the action of water. They present much the same appearance

as formations on our own earth which we know have been fashioned by that

means. There is no water upon the moon now, I think, though several

large depressions are still called oceans, seas, lakes, or marshes,

because at one time they were believed to be such. Probably in some of

those places, if not in all, water existed millions of years ago; but

ages since they must have lost it either by evaporation or by absorption

into the soil.



"I will not say any more just now, but as we pass above the lunar

surface I will point out a few of the natural features that may be of

interest to you."



M'Allister here paid me the compliment of saying, "Well, Professor, I

always thought astronomy was a very dry and difficult subject; but your

remarks were really very interesting, and quite easy to understand.

There is only one thing that seemed to me rather strange as coming from

a scientific man, and I would like you to explain that."



"Certainly; if there is anything you do not quite understand, you have

only to ask and I will try to clear the matter up," I answered. "What

is it you wish to know?"



"Well," he answered, "I noticed that when you were speaking about the

distance of the moon you always said it was about so far away. Why

didn't you tell us the exact distance? I'm not a scientific man by any

means, but if any one were to ask me the length of a connecting rod on

one of my machines I should say '25 inches,' not 'about 25 inches,' for

that would not do for a practical man!"



"It's like this, M'Allister," I said. "You measure things with a

two-foot rule, which is something you can actually handle, and you know

it is made according to a standard measure and must contain exactly 24

inches. If, however, your rule was 24-1/4 inches long, yet still divided

into twenty-four equal parts, you could measure work with it just the

same, but would know that every measurement was just a little bit out.

If you had no possible means of obtaining another rule, you would have

to put up with a little inexactitude.



"That is just the position in which astronomers are placed; they have to

put up with a measure which they know is not perfectly accurate, yet it

is the best which can be secured.



"Their two-foot rule, so to speak, may be the distance from the earth to

the sun, or the length of the whole diameter of the earth's orbit, and

these cannot be handled like your rule; and although we know the

measurements of these are nearly correct, they are not quite so. Yet the

distances of the moon, planets, stars, &c., have to be measured by these

rules, so it is clear we can only know those distances with a near

approximation to accuracy.



"For this reason astronomers are always trying different means of

ascertaining the sun's exact distance from the earth in order to obtain

a perfectly correct measure; but there are so many difficulties and

complications which affect the result, that it will be a long time yet

before they succeed in their work.



"You will therefore understand that all these figures as to distances

and dimensions of planets and stars are only as near approaches to

correctness as is possible to attain in our present circumstances. They

must not be regarded as literally exact, although they are usually

sufficiently accurate for all general purposes. Astronomers know this

and allow for it; but general readers of books, when they find figures

which do not agree with others they have seen, are apt to regard them as

all being mere guesses, and in this they are doing an injustice to the

painstaking labours of generations of astronomers and mathematicians.



"I shall presently be mentioning the heights of mountains, the size of

ring-plains, craters, &c., but the same reasoning applies to them; the

dimensions given are averages of measurements made by different

observers, and, though not quite accurate, are as near the truth as the

difficult conditions under which they have to be measured will allow."



"Thank you, Professor," said M'Allister as I concluded. "I'm glad I

don't have to work with such rules as those you mention, for

measurements a little bit out of correctness would ruin any machine in

the world."



"Still, M'Allister," I said, "you would have the advantage over

astronomers with your two-foot rule, because you would know that it was

a quarter of an inch too long. Their difficulty is that they do not

know exactly how much their rule is out of correctness, so cannot obtain

absolute accuracy however they may try."



We now set the machines going very slowly and moved toward the northern

part of the moon, where I pointed out the position of the lunar north

pole, and explained that, owing to the very slight inclination of the

lunar axis, there can be but very little variation of seasons in any one

particular part of the moon. Thus, if at one place it were spring, it

would practically always be spring there, but with very cold nights all

through the lunar year. Where it was summer it would practically always

be summer, also with very cold nights, and so on.



I further explained that, as the moon revolves on its axis in the same

time that it takes to make one revolution round the earth, those on the

earth always see the same side of the moon, except when occasionally,

owing to inequalities in the lunar motions, they are afforded a peep

just round portions of the edges at different periods. The remainder of

the other side of the moon has never yet been seen from the earth by

human eyes, and in all probability never will be seen for millions of

years to come.



John, who as usual was smoking like a factory chimney, here removed his

pipe from his mouth and said, "Professor, you stated just now that the

nights on the moon would always be intensely cold, and I should like to

know whether there is any really reliable information respecting the

temperature of the lunar days and nights. I have seen so many

contradictory theories on the subject that I scarcely know what to

believe."



"In that respect," I answered, "I fear that my position is much the

same as yours, for I have absolutely no certain knowledge on the point,

but will just state shortly how the matter stands at present.



"During the past century many investigations have been made by

scientific men respecting the temperatures on the moon, and their

results have differed to an amazing extent. It would take too long, and

be too wearisome, to quote all the authorities, so a few must suffice.

Lord Rosse, who used a thermopile in his experiments, found that in

order to produce the results he obtained, the sunlit surface of the moon

must be heated to a temperature of 500 degrees on Fahrenheit's scale.

Sir John Herschel had previously concluded that the temperature must be

much greater than that of boiling water. On the other hand Ericsson and,

more recently, Professor Langley-who used a bolometer of his own

invention for measuring the heat of the sun's invisible rays-came to

the conclusion that even under continued sunshine the temperature would

rarely, if ever, rise above the freezing point of water.



"Professor Very, however, who has continued the delicate experiments

with the bolometer, and also made other experiments and calculations of

quite recent date, has obtained results more nearly in accordance with

those first quoted, for he found that near the end of the second week's

sunshine on the moon the temperature of the rocks, soil, &c., must rise

to at least 80 degrees Centigrade above the heat of boiling water."



"My word!" said M'Allister, "that's hotter than a ship's engine-room,

and I shouldn't care for such a very high temperature."



"As this is so recent," I proceeded, "and the work of one of our

highest authorities, I think we must accept it as being more correct,

especially as Professor Very has taken into consideration some factors

which had not previously been allowed due weight.



"In connection with this matter of temperature it is necessary to

remember that the days and nights upon the moon are both very long, for

the full lunar 'day' is equal to a month, so the actual lunar day is

equal to fourteen of our days, and the lunar night is of the same

duration. Our 'day' of twenty-four hours is divided into day and night

in unequal proportions, according to the changes in the seasons; but, as

I before remarked, the seasonal changes on the moon are very slight, so

the variations in the lengths of the days and nights are very small.



"But, whatever may be the difference of opinion as to the heat of the

lunar day, there seems to be a pretty general agreement that, owing to

the absence of an atmosphere, the nights must be so intensely cold as to

be almost beyond our conception-probably approaching nearly to the

absolute zero of outer space. Even with an atmosphere the long nights in

our polar regions are so cold that only very strong people can endure

them, notwithstanding every device for obtaining warmth.



"You will gather from this that although the moon appears so beautiful

from a distance, it must be anything but a desirable place of residence

even from a climatic point of view, for we should practically be fried

at midday, while at midnight-or even in the daytime when out of the

direct rays of the sun-we should soon be frozen stiff."



As I said this John chimed in with: "Professor, all things considered,

I think I could smoke my pipe more comfortably upon the earth than upon

the moon. I really don't like such extremes of temperature."



"I am of the same mind," I replied, "and it is because I prefer a more

equable temperature that I have carefully kept our martalium blinds

drawn over those windows of our vessel upon which the sun is shining."