. The Americana : a universal reference library, comprising the arts and sciences, literature, history, biograhy, geography, commerce, etc., of the world . ? case the upper part of the freshexcavation is protected by the vizor exten-sion of the shield. Where tlie material isstiff enough to hold itself up for a shorttime the front extension of the shell is notneeded. It is thus seen that the purpose of the is needed for the material at the bottom ofthe tunnel than at the top, but as the pres-sure required for the bottom must in generalbe used, there will be danger of blow-outsat the top, requir
. The Americana : a universal reference library, comprising the arts and sciences, literature, history, biograhy, geography, commerce, etc., of the world . ? case the upper part of the freshexcavation is protected by the vizor exten-sion of the shield. Where tlie material isstiff enough to hold itself up for a shorttime the front extension of the shell is notneeded. It is thus seen that the purpose of the is needed for the material at the bottom ofthe tunnel than at the top, but as the pres-sure required for the bottom must in generalbe used, there will be danger of blow-outsat the top, requiring great caution in softmaterial. With a diameter of shield of 22feet the pressure at the bottom of the excava-tion may exceed the water pressure at thetop by between g and lo pounds per squareinch. Such inequalities tend to produceeither a blow-out at the top of the headingor an inrush of water and soft material atthe bottom. The table at the top of the follow-ing page was compiled by Mons. R. Le Gouezand shows the dimensions of the cylindricalshells of some of the principal shields used intunneling through soft material up to the pres-ent Cross Section. Fig. 8. shield is to prevent the inrush of water andsoft material while excavation is being made,the diaphragm of the shield acting as a bulk-head and the openings in it being so devisedas to be quickly closed if necessary. Theextension of the shield in front of tlie dia-phragm is designed to prevent the falling orflowing in of the exposed face of the newexcavation. The extension of the shell backfrom the diaphragm is designed to affordopportunity for putting in place the finishedtunnel lining, whether of cast iron or of steelplates lined with masonry. Where the mate-rial is saturated with water it is usually neces-sary to use compressed air in connection withthe .shield. The intensity of this air pressureis determined by the depth of the tunnel be-low tlic surface of the water above it. It isevident that a greater int
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