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he altitude of the pole； or the latitude of you； the observer。
〃Of course we know this；〃 says the graduating world。 〃Do you suppose that is what we borrow your book for； to have you spell out your miserable elementary astronomy？〃 At which rebuff I should shrink distressed； but that a chorus of voices an octave higher comes up with； 〃Dear Mr。 Ingham； we are ever so much obliged to you； we did not know it at all before； and you make it perfectly clear。〃
Thank you； my dear； and you； and you。 We will not care what the others say。 If you do understand it； or do know it； it is more than Mr。 Charles Reade knew； or he would not have made his two lovers on the island guess at their latitude； as they did。 If they had either of them been educated at a respectable academy for the Middle Classes； they would have fared better。
Now about the longitude。
The latitude； which you have found； measures your distance north or south from the equator or the pole。 To find your longitude； you want to find your distance east or west from the meridian of Greenwich。 Now； if any one would build a good tall tower at Greenwich； straight into the sky；say a hundred miles into the sky；of course if you and I were east or west of it； and could see it； we could tell how far east or west we were by measuring the apparent height of the tower above our horizon。 If we could see so far； when the lantern with a Drummond's light； 〃ever so bright；〃 on the very top of the tower； appeared to be on our horizon； we should know we were eight hundred and seventy…three miles away from it。 The top of the tower would answer for us as the North Star does when we are measuring the latitude。 If we were nearer； our horizon would make a longer angle with the line from the top to our place of vision。 If we were farther away； we should need a higher tower。
But nobody will build any such tower at Greenwich； or elsewhere on that meridian； or on any meridian。 You see that to be of use to the half the world nearest to it； it would have to be so high that the diameter of the world would seem nothing in proportion。 And then； for the other half of the world you would have to erect another tower as high on the other side。 It was this difficulty that made Q。 suggest the expedient of the Brick Moon。
For you see that if； by good luck； there were a ring like Saturn's which stretched round the world； above Greenwich and the meridian of Greenwich； and if it would stay above Greenwich； turning with the world； any one who wanted to measure his longitude or distance from Greenwich would look out of window and see how high this ring was above his horizon。 At Greenwich it would be over his head exactly。 At New Orleans； which is quarter round the world from Greenwich； it would be just in his horizon。 A little west of New Orleans you would begin to look for the other half of the ring on the west instead of the east； and if you went a little west of the Feejee Islands the ring would be over your head again。 So if we only had a ring like that； not round the equator of the world；as Saturn's ring is around Saturn；but vertical to the plane of the equator； as the brass ring of an artificial globe goes； only far higher in proportion； 〃from that ring；〃 said Q。； pensively； 〃we could calculate the longitude。〃
Failing that； after various propositions； he suggested the Brick Moon。 The plan was this： If from the surface of the earth； by a gigantic peashooter； you could shoot a pea upward from Greenwich； aimed northward as well as upward； if you drove it so fast and far that when its power of ascent was exhausted； and it began to fall； it should clear the earth； and pass outside the North Pole； if you had given it sufficient power to get it half round the earth without touching； that pea would clear the earth forever。 It would continue to rotate above the North Pole； above the Feejee Island place； above the South Pole and Greenwich； forever； with the impulse with which it had first cleared our atmosphere and attraction。 If only we could see that pea as it revolved in that convenient orbit； then we could measure the longitude from that； as soon as we knew how high the orbit was； as well as if it were the ring of Saturn。
〃But a pea is so small！〃
〃Yes；〃 said Q。； 〃but we must make a large pea。〃 Then we fell to work on plans for making the pea very large and very light。 Large；that it might be seen far away by storm…tossed navigators： light；that it might be the easier blown four thousand and odd miles into the air； lest it should fall on the heads of the Greenlanders or the Patagonians； lest they should be injured and the world lose its new moon。 But； of course； all this lath… and…plaster had to be given up。 For the motion through the air would set fire to this moon just as it does to other aerolites； and all your lath…and…plaster would gather into a few white drops； which no Rosse telescope even could discern。 〃No；〃 said Q。 bravely； 〃at the least it must be very substantial。 It must stand fire well； very well。 Iron will not answer。 It must be brick； we must have a Brick Moon。〃
Then we had to calculate its size。 You can see； on the old moon； an edifice two hundred feet long with any of the fine refractors of our day。 But no such refractors as those can be carried by the poor little fishermen whom we wanted to befriend； the bones of whose ships lie white on so many cliffs； their names unreported at any Lloyd's or by any Ross；
Themselves the owners and their sons the crew。
On the other hand； we did not want our moon two hundred and fifty thousand miles away； as the old moon is； which I will call the Thornbush moon； for distinction。 We did not care how near it was； indeed； if it were only far enough away to be seen； in practice； from almost the whole world。 There must be a little strip where they could not see it from the surface； unless we threw it infinitely high。 〃But they need not look from the surface；〃 said Q。； 〃they might climb to the mast…head。 And if they did not see it at all； they would know that they were ninety degrees from the meridian。〃
This difficulty about what we call 〃the strip；〃 however； led to an improvement in the plan； which made it better in every way。 It was clear that even if 〃the strip〃 were quite wide； the moon would have to be a good way off； and； in proportion； hard to see。 If； however； we would satisfy ourselves with a moon four thousand miles away； THAT could be seen on the earth's surface for three or four thousand miles on each side； and twice three thousand； or six thousand； is one fourth of the largest circumference of the earth。 We did not dare have it nearer than four thousand miles； since even at that distance it would be eclipsed three hours out of every night； and we wanted it bright and distinct； and not of that lurid； copper； eclipse color。 But at four thousand miles' distance the moon could be seen by a belt of observers six or eight thousand miles in diameter。 〃Start； then； two moons；〃this was my contribution to the plan。 〃Suppose one over the meridian of Greenwich； and the other over that of New Orleans。 Take care that there is a little difference in the radii of their orbits； lest they ‘collide' some foul day。 T