

REVIVOR. 



REVOLVERS. 



06 



equal to half the pressure, we have | (ei-f ) for the effect of friction ! 

 and in the case of equilibrium, I H = R n+ J (oi + s B). 



Let EC = /<, ED-r, H8=p,and let y be the specific gravity of the 



earth ; then - expresses the weight of the prism whose section is 



IDC, and whose thickness is unity, and which was represented by o H ; 

 and the triangles o I H, H s B being similar to E c n, we get by propor- 

 tions i H =*-?, HB=*, ci=^L, andsB = A. These values 



2CD CD 2CD CO 



being substituted in the above equation, the value of the pressure B s 



or p will be found to be y 



21,:-:- 



Now this quantity is to be 



2z + A 



a maximum ; therefore making its differential relatively to : equal to 

 zero, the value of z will be found to be'GlSA; whence/) = '1908 jr. A 5 . 

 If this equation be differentiated relatively to A, the result will express 

 the horizontal pressure against an elementary portion of the wall at a 

 variable height (which represent by A) above C : therefore multiplying 

 by A and integrating, we get -1272 y k 3 for the whole force exerted by 

 the earth to overturn about M a wall whose height E o is represented 

 by A, when that force is a maximum. When there is a parapet above 

 A D, its weight, expressed by the product of the area of the section 

 multiplied by .'/, must be added to the above expression for the weight 

 of the prism E o c in the preceding investigation, in order to obtain 

 the value of the expression which is represented by .. K. 



Instead of making a revetment in the form of a simple wall, it U 

 customary to build buttresses or counterforts at intervals from each 

 other on the side next to the supported earth ; consequently the 

 thickness of the wall iUelf may be rather less than that which would 

 result from the above equation. In order to determine it, if we 

 assume, for example, that the distance from the centre of one counter- 

 fort to that of the next is fifteen feet, the area of a horizontal section 

 fifteen feet long and taken at the mean height of the wall, if the face 

 has a slope, together with the area of the like section through the two 

 half-counterforts, may be equal to the area of a section of the simple 

 wall (of the same length) as determined by the above investigation ; 

 oe-fifthoftT 



. 



\ of that quantity fur the two half -counterforts, 



the remainder divided by 15 will give the required breadth of 

 horizontal section at the part between the counterfort*. It should be 

 observed, however, that the thickness of a brick revetment which is to 

 resist the fire of a battering-train should not be lees than seven or 

 eight feet. It is usual to make the depth of a counterfort equal to the 

 mean breadth of the wall ; and to give it greater thickness at the part 

 which joins the wall than at the other extremity. 



Counterforts serve in part as props to keep the wall from inclining 

 hi consequence of partial compressions in the cement ; but chiefly by 

 extending the breadth of the base at intervals they increase, at the 

 places where they are formed, the length of the arm of the lever by 

 which the weight of the wall resists the lateral pressure of the earth. 

 The usual connection of the bricks or stones in the wall with those of 

 the counterforts allows this advantage to be extended in nearly an 

 equal degree to the parts which are situated between the counterforts. 

 But in order that the connection may be more complete, it has been 

 recommended to connect the tails of every two counterforts by a wall 

 curved on the plan, and having the convex side towards the earth 

 which is to be retained. Again, the nearest sides of every two counter- 

 forte have occasions lly been connected by two or more arches, one 

 above another ; by which 



e e 



supported, and the lateral 

 Revetments in which 



the mass of retained earth U in part 



of the whole is diminished. 

 counterforts are connected in either of 



those ways are said to be counter-arched ; and it U recommended that 

 arches of the latter kind should be formed in the mass of the parapet 

 above the cordon of the scarp. It is also recommended that the 

 mason 17 of the arches in the rampart should be but slightly connected 

 with that of the revetment wall ; since the greater part of the rampart 

 and parapet will then remain supported by the arches even after the 

 revetment has been demolished by the artillery of the besiegers. 

 When the escarp revetment is carried completely up to the superior slope 

 of the parapet, leaving no exterior slope, it is termed a full .1 1 miftsjsaf. 

 When there is an exterior slope left it Is termed a dtmi-rtrrtmnl. 



REVIVOR. (Sere* fACiAi.1 



RE Vi >LL'TION. Thii well-known term u applied in astronomy to 

 the manner in which a detached body moves round another, as a 

 planet round the sun ; but the motion of connected particles of matter 

 round an axis, such as the diurnal revolution of a planet, U more 

 usually called ROTATION. 



In pure mathematics the word revolution it applied to the angle moved 

 over by a line which revolves round a point from any one position to 

 that position again ; it is therefore a synonyme for four right angles. 



KKV< il.VEKS are fire-arms in which the barrels, or a portion of the 

 barrels, being made to turn round on the cocking of the piece, are 

 brought successively under the hammer, and permit of a rapid succes- 

 sion of discharges from the same piece. Revolver*, or repeating fire- 

 arms, are not an invention of modern times ; many of a very ancient 

 date are to be seen in museums and collections of old arms. These are 

 hi several points very similar to the most recent improveroente, but it 

 was not till the introduction of the percussion cap removed the 



difficulty of retaining the priming of each 

 ARTS AJTD SCI. BIT. VOL. VII. 



place during 



the rotation of the chamber, that the revolver'could be considered a 

 really serviceable weapon ; while at the same time it wag not till the 



fig. 1. , the chamber; B, the barrel, rifled; c, the levor-ramrod. 



great improvement of modern times in cheap accurate machine manu- 

 facture that it became practicable to make such instruments in large 

 numbers. For many years revolvers were made with the barrels all in 



Fife. J A, the barrel ; t, cylinder or chamber ; c, lerer-ramrod. 



one ; five or six barrels being fixed together were made to revolve round 

 a central axis, so as to bring their nipples successively under the hammer. 



