"A zimilar remarg was made nod zo very many years ago when id was suggesded that ocean shibs could be buildt of medal," retorted the professor. "Yed there are thousands of medal shibs in exisdenze do-day; and there can be no doubt as do the facd thad they fload. And zo will an aerial shib. The gread--in facd the _only_ diffiguldy in the madder is thad air is eight hundred dimes lighder than wader; and an air shib of given dimensions musd therefore be ad leasd eight hundred dimes lighder than her ocean sisder do enable her do fload in the atmosphere. The broblem, then, is this: How are you to gonsdrugt a medal shib, of given dimensions, sdrong enough do hold dogether and withsdand the shock of goming do earth, yed of less weighd than her own bulk of air? With the medals hitherdoo ad our disbosal, I admid thad the dask is a diffiguld one; bud I maindain thad id is by no means an imbossibilidy. An ocean shib musd be buildt sdrong enough nod only do susdain the weighd of her gargo--often amounding do upwards of a thousand dons--bud also do withstand the dremendous and incessandly varying sdrain do which she is exbosed when garrying thad gargo through a moundainous sea. This enormous sdrength necessidades the use of a gorresbonding thickness--and therefore weighd--of the medal used in her gonsdruction. Such brovision would of gourse be unnecessary in the gase of an aerial shib; begause no one would dream of garrying an ounze of unnecessary weighd through the air; and there are no moundain seas in the admosphere to sdrain a shib. A vasd saving in weighd would resuld from these zirgumsdances alone; and a further saving--zufficiend, I believe, to aggomblish the desired object--gan, no doubd, be effecded by skilful engineers, one of whose greadesd driumphs id is do design sdrugdures in which the maximum of sdrength is zecured with the minimum of weighd. Id musd nod be forgodden, either, thad an air shib musd, in one imbordand bardigular, be dreated exactly like her ocean sisder. An ocean shib gonsdrugded, say, of sdeel, will sink if filled with wader, begause sdeel is heavier than wader, bulk for bulk; bud bump oud all the wader from her inderior, and if she be proberly gonsdrugded, she will fload on the elemend she is indended do navigade. And the same with an air shib: bump out all or nearly all the air which she gondains, and if she be gonsdrugded in aggordanze with the brincibles I have indigaded, she will fload in the lighder elemend." "Upon my word, professor, you have argued your case extremely well," exclaimed the colonel. "I can see only one difficulty in the way; and that is in the matter of _weight_."