REVKRSJON. 



REVETMENT. 



Unity Ma. 



to reversion. There may or may 

 always do* from UM owner of UM 

 r. Bad it cannot b* separated from 



lor years ari 

 wi* 



bject to 



rent, if there be 

 rsnioas which are 

 

 oa a freehold estate. 



. : 



Whca a revenioa 

 it i. babi. to UM 







. - - 



by th.. who were at any time entitled 

 aate contained in such leases. All 

 U tail. are subject to merge in UM 

 the particular sstste and the reversion are united in 

 tit* eame person. Formerly when an estate tail wa* converted into a 

 lc fee. and the remainder or reversion in fee in the name land* 

 became united in UM aune person. UM ban fee wmi subject to merger 

 in UM reversion : but by the S and 4 Wm. IV. c. 74. when auch union 

 takes place. " UM ban lee ehall not merge, but shall be y*> faeto 

 enlarged into a* large aa Mate at the tenant in tail, with the content 

 el UM protector, if any. might hare created by any disposition under 

 thU Act, if such remainder or rereraoa had been vetted in any othtr 

 psnon.- Before tab statute, when a ban (ee thus merged in the 

 leieiaiua, the reversion became an estate in paseeesion, and liable to 

 all UM leases and charges of thoee who had at any time been entitled 



opposite aide of the ditch. The exterior faces of then walla are con- 

 sidered as the scarp and counterscarp of the ditch. 



In and before the time of Vanban the scarp revetments were raised 

 from the bottom of the ditch to the top of the parapet; but the part 

 whkh was risible above the glacis bemg destroyed by the enemy's 

 artillery, and the parapet in consequence partly ruined soon after th- 

 commencement of the siege, that engineer in meet of his works raised 

 his revtiUMBts BO higher than the level of the crest of the glacis, or 

 about 7 (set above the natural ground; the exterior of the parapet 

 was then left at such an inclination to the horizon (45* in general) that 

 UM earth would support itself. The ditch of a fortress being about IS 

 net deep. UM height of the scarp revetment was consequently 25 feet, 

 and this was considered sufficient to afford security against the danger 

 of having the rampart eecaladed. At present it is recommended that 

 the main ditch should be 24 feet deep, and in this case the scarp 

 revetment ia above SO feet high. In constructing the fortifications of 

 Ncuf Brisae, Vauban made the revetments of the scarps both of the 

 enceinte and of the reduit of the ravelin, as high as the top of the 

 parapet ; but then works being covered by the counter-guard or the 

 ravelin, their revetment* would be unseen by the enemy at a distance, 

 and therefore not liable to the objection above mentioned. 



The form usually given in profile to revetments of masonry may be 

 seen at * and s, fty. 2, BASTIOK ; the first ia the revetment of the 

 counterscarp, and the other that of the scarp. The rectangular parts 

 are sections through the counterforts or buttresses which are built up 

 with the walla in order to strengthen them, at intervals of about 15 feet 

 from each other. Scarp revetment*, whose tops are as high or higher 

 than the crest of the glacis, are called full revetment* ; while such 

 as are no higher than the level of the natural ground are called demi- 



In order that the revetment might most effectually resist the 

 pressure of the earth which it is to support, Vauban gave to the 

 exterior face of the wall a slope, whose horizontal breadth was equal to 

 one-fifth of the height ; this was subsequently reduced to one-sixth, 

 and now it is thought to be most advantageous to make the face 

 vertical. By giving the slope to the rear face of the wall, that is, by 

 r** 1 ' 5 '^; the revetment counterslopini;, all the advantages of a slope are 

 gained, while the disadvantage of rapid vegetation on the face is 



In laying the foundations of revetments in defective soil*, the same 

 methods are used as in the construction of civil edifices; and in all 

 cans the courses of stones or bricks are laid obliquely to the horizon, 

 *~^* h ^f down towards the part under the earth which is to be sup- 

 ported, in order that the pressure of the Utter may be more directly 

 resisted. But as the bed-joints of brickwork when so disposed allow 

 the rain to penetrate, and the seeds of grass to lodge in them, it is 

 thought that the wall is more speedily degraded when so built than when 

 UM courses are hud horizontally; therefore in order to unite the 

 advantages f direct resistance and durability, it is customary to place 

 the courses obliquely, but to lay one row of bricks in each course at 

 the (ace of the wall in a horizontal poi: 



The exterior and interior faces of the revetment, or retaining wall of 

 a dock, have in a vertical section the form of concentric arcs of circles, 

 with their convexities towards the land ; and this form is given them 

 that the atones may be able to resist the hydrostatical pressure of any 

 water which, when the dock is full, may get behind the wall, and which 

 nted from escaping when the dock is made dry. 



may be pi 



of the ramparts of Coehorn, and all of thoee which Carnot 

 proposed for his fortresses, are formed of earth unsupported by revet- 

 ments ; and even the opposite aide of the ditch, instead of being faced 

 with a steep wall, is by the latter engineer cut with a gentle dope from 



UM lev*! of the natural ground to the bottom of the ditch. But the 

 foiliflciUrsai of Coeborn are provided with wet ditches, which prevent 

 the Ismugfrs from getting to the foot of the rampart by surprise ; and 

 in those of Carnot a high detached wall covered by a countrguard of 

 earth puts it out of the power of the enemy, while that wall stands, to 

 get across the ditch. Without such obstacles the unreveted rampart 

 would afford great facilities to the enemy in an effort to carry the 

 foitiees by assault. Its exterior alope must form at moxt an angle of 

 45* with the horizon, that the earth may support itself, and con- 

 sequently it may be easily ascended; and any palisades or other 

 impediments which the defenders might place on it would soon be 

 displaced or destroyed by the batteries of the enemy. Besides these 

 evils, the exterior slope, from its breVHh, occupies a great portion of 

 ground ; it consequently obligee the engineer to contract the space 

 enclosed within the works, and thus to sacrifice in some measure the 

 convenience both of the inhabitants and the garrison. 



In order to investigate the conditions of stability in revetment walls, 

 let BBC be a vertical section through the man of earth retained by the 

 wall ; B c being the slope which earth is supposed to assume when 

 unsupported, and let A E M K be a section of the wall, e c being the 

 level of the bottom of the ditch, and M x being the bottom of the 

 foundation. Imagine o to be the centre of gravity of the section E CB ; 

 draw o L parallel to B c and o E parallel to the horizon : then, by the 

 resolution of forces, E L and c K will have to one another the same 

 proportion that the weight of the unsupported prism of earth (of any 

 thickness) bean to its horizontal pressure. Let w be that weight ; then 



ICQ KO 



- will express that pressure, and .w . K X will be the momentum 



or power by which the earth tends to overturn the wall about y. 



a * 



Imagine the vertical line A o. to be drawn ; then the form and dimensions 

 of the part A My of the wall are known ; and let it be required to find 

 the breadth g x of the rectangular part A x , so that the resistance of 

 the whole shall be equal to the momentum of the supported earth. 

 Suppose the centre of gravity of A 11 Q to be found, and let it 1 v 

 vertically over a. The centre of gravity of the rectangular part is 

 vertically over b, the middle of Q x ; and let Q 6 be represented by x. 

 Then if y be the specific gravity of the wall, we have by mechanics, 

 iM<t.ua.g+&(t.Ml>.g.x for the resistance of the wall ; conse- 

 quently equating this expression with the above momentum of the 

 earth, the value of x, and therefore of Q x, can be found. But great 

 uncertainty exists respecting the position of the line of rupture B c, 

 from our ignorance of the allowance to be made for the effect of 

 friction on the tendency of the earth to slide downwards. Experiments 

 have led to the opinion that this effect is equal to half the pressure of 

 the earth perpendicularly upon the inclined plane which it would 

 assume if unsupported ; and that value is frequently adopted. Coulomb 

 showed that the angle which the line of rupture makes with the 

 vertical is half the angle which the line of natural slope makes with 

 the vertical. 



In order to find the magnitude which the triangle E D c should have 

 when the supported earth exerts the greatest pressure against the wall, 

 the following process may be used ; the earth above A D being at present, 

 for simplicity, supposed to be removed. Imagine c to be the centre of 

 gravity of that triangle, aud the vertical line c H to be drawn ; then 

 B may represent the weight of the unsupported earth, and let it be 

 resolved into the pressures represented by c I and I B, the former per- 

 pendicular to the alope, and the latter coincident with it Imagine 

 H s to be drawn to represent the reaction of the wall A si x, and let it 

 be resolved into the forces represented by s B and K B, perpendicular 

 to and coincident with the elope, respectively. Then, I H representing 

 the force with which the prism of earth would tend, if without friction, 

 to slide down D c, R B represents the reaction by which the wall resists 

 that force ; while Q I and 8 B represent the pressure and reaction per- 

 pendicular to DC. Consequently, the friction being supposed to be 



