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to support the beams by struts to the piers, thanby trusses.

cases the failures that took place in the attempts to destroystone arches, arose from not applying the mines in a propermanner. These failures and their causes having become mat-ter of discussion, in which I took part, on the voyage fromCoruna , and being then preparing materials for the first edi-tion of this work, I noted my observations upon this impor-tant matter, and sent them to the late Dr. Hutton for hiscorrection and opinion.—I cannot do better then annex theanswer which I received from that eminent person.

“ It was with great pleasure I received your letter, both asfrom an old pupil and friend, and as a friend safely returnedfrom so dangerous a situation in a foreign country. As tothe failure in the attempts to destroy the bridges, it is not tobe wondered at, considering the rapidity of the retreat, a cir-cumstance which would alone induce to sink the mine fromabove: that being the part of least resistance, the consequencemust be, the blowing an excavation upwards, which could bereadily repaired again by filling in rubbish, like fdling up ahole in the high road. If time and convenience would allow—to make an effectual breach, it would seem best to makethe mine horizontally in the side or face, either in or over apier, or in the flank, or even under the crown of the arch.In the first of these cases, in or over a pier, the pier servingas a firm basis, the explosion would be chiefly upwards, andwould be large and deep, and might, or might not, let downthe adjoining arch or arches. In the other two cases, indriving the mine either in the flanks or crown of the arch, itseems that it would be well to keep it pretty near, or butlittle above the curve or intrados of the arch, and as farbelow the upper surface or road-way as possible; so that