Sir,--In reply to Colonel Jackson's inquiry as to any rule of international law bearing upon aerial attack upon London, I referred him to the, now generally accepted, prohibition of the "bombardment, _by any means whatever_, of towns, &c., which are not defended." This rule has been growing into its present form ever since the Brussels Conference of 1874. The words italicised were added to it in 1907, to show that it applies to the action of _aéronefs_ as well as to that of land batteries. It clearly prohibits any wanton bombardment, undertaken with no distinctly military object in view, and the prohibition is much more sweeping, for reasons not far to seek, than that imposed by Convention No. ix. of 1907 upon the treatment of coast towns by hostile fleets. So far good; but further questions arise, as to which no diplomatically authoritative answers are as yet available; and I, for one, am not wise above that which is written. One asks, for instance, what places are _prima facie_ "undefended." Can a "great centre of population" claim this character, although it contains barracks, stores, and bodies of troops? For the affirmative I can vouch only the authority of the Institut de Droit International, which in 1896, in the course of the discussion of a draft prepared by General Den Beer Pourtugael and myself, adopted a statement to that effect. A different view seems to be taken in the German _Kriegsbrauch_, p. 22. One also asks: Under what circumstances does a place, _prima facie_, "undefended," cease to possess that character? Doubtless so soon as access to it is forcibly denied to the land forces of the enemy; hardly, to borrow an illustration from Colonel Jackson's letter of Thursday last, should the place merely decline to submit to the dictation of two men in an aeroplane. I read with great pleasure the colonel's warning, addressed to the United Service Institution, and am as little desirous as he is that London should rely for protection upon The Hague article, ambiguous as I have confessed it to be; trusting, indeed, that our capital may be enabled so to act at once in case of danger as wholly to forfeit such claim as it may in ordinary times possess to be considered an "undefended" town. Let the principle involved in Art. 25 be carried into much further detail, should that be found feasible, but, in the meantime, let us not for a moment relax our preparation of vertical firing guns and defensive aeroplanes. I am, Sir, your obedient servant, T. E. HOLLAND. Oxford, May 2 (1914). The war of 1914 has definitely established the employment of aircraft for hostile purposes, and, as evidenced by the reception given by belligerents to neutral protests, the sovereignty of a state over its superincumbent air-spaces. On the bombardment of undefended places, _cf. supra_, pp. 30, 62, 67, 68; _infra_, pp. 97, 109, 112-123.