uracy.

"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