Colonel Wethered further communicated to _The Times_ of May 25, 1877, a letter on "Portable Batteries" in which he declared that if, before an enemy could effect a landing, we were to provide the means of concentrating, with unerring certainty, on any given points of the coast, a crushing force of artillery, with guns of heavier calibre than even the warships of the invader could command, it would be impossible for the vessels of an invading force to approach near enough to effect the landing of their men. He continued:-- My proposal is to take the full advantage which our railway system, in connection with our insular position, affords, and provide powerful moveable batteries which can be sent fully equipped in fighting order direct by railway to any required point; and the recent experimental trials of the 81-ton gun have proved that the heaviest ordnance can be moved and fought on railway metals with considerable advantage.... In connection with our present main lines of railway, which probably would require strengthening at certain points, I would construct branch lines or sidings leading to every strategical point of our coast and into every fort, as far as possible, with requisite platforms.... These branch lines during peace would, doubtless, be of some small commercial value.... I would mount as many of our heaviest guns as practicable on railway gun carriages so that they could be moved by rail from one face of a front to another, and from one place to another. He also recommended that guns thus mounted, fully equipped, and ready for use, should be kept at three large central depôts which might be utilised for the defence of London. At each of them he would station (1) Militia and Volunteer Artillery able not only to work the guns but to construct, repair or destroy railway lines, and (2) a locomotive corps specially trained in the working of traffic under war conditions.