"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.