516 



MISCELLANEOUS GARDEN STRUCTURES. 



any common arch may be built without 

 centering as far up as where the courses 

 lie at an angle of about 32°, or what is 

 called the angle of repose for masonry — 

 that is, where the bricks will first begin 

 to slip off; but a brick dome may be 

 built of any size entirely without center- 

 ing, for the following reason : — Referring 

 to fig. 732, dd are two bricks supposed to 

 belong to part of the course of bricks 



Fig. 732. 



//W Fig. 732*. 



next above that of the angle of repose. 

 Each of these is to be considered, with 

 the mortar in which they are embedded, 

 as a quadrangular prismatic frustrum, 

 whose sides all incline towards the centre 

 of the hemisphere at c. Now, the upper 

 surfaces of these two bricks form an in- 

 ternal or retaining angle with one another, 

 from the position they lie in on the pre- 

 ceding courses : that is, they lean against 

 each other as if they lay on opposite in- 

 clined planes, as shown in fig. 732*. If, 

 then, these bricks slip, they must do so in 

 the line e f ; but, in doing so, they must 

 approach each other. But they are 

 already in contact, therefore they cannot 

 slip. This demonstration applies to any 

 greater number of bricks until the whole 

 course is finished, when the bricks are 

 sustained by their lateral thrust. There 

 is a limit to the weight of the voussoir, 

 (the overhanging part of an arch looking 

 up from under it,) which will support 

 itself in this way, as must be obvious to 

 every one from the common principles of 

 gravitation. It is also obvious that a 

 dome may thus either be left open or 

 closed at top. To make the tank per- 



fectly water-tight, it is finally coated over 

 two or three times with coal-tar inside. 

 A man-hole is shown at g, fig. 731, for 

 getting in to clean it out occasionally." 

 A great deal of the success of all tanks 

 depends on the external resistance being 

 equal at all points, for the pressure of 

 fluids is great ; and if one part of the re- 

 sisting soil or embankment yield, the 

 building, however strong, is apt to follow. 

 Hence spherical or circular forms are 

 better than those with straight sides, at 

 least so far as strength is concerned. 



In forming tanks or reservoirs for the 

 propulsion of water, it is necessary to as- 

 certain the pressure the tank has to sus- 

 tain, the more especially if the tank be 

 above ground, and have no resisting pres- 

 sure from without. The rule for ascer- 

 taining this is laid down in the " Phar- 

 maceutical Times," as follows: — "The 

 pressure on the bottom of the tank is 

 equal to the whole weight of the fluid it 

 contains, as none of the weight is sup- 

 ported by the vertical sides. The amount 

 is found by multiplying the area of the 

 base in feet by the altitude in feet, and 

 the product, which is the number of cubic 

 feet it contains, by 62.23, which is the 

 number of pounds weight of a cubic foot 

 of water, giving the total weight in pounds. 

 The entire lateral pressure, or that on 

 the sides in a cylinder, is to the weight of 

 the fluid as the altitude is to the radius of 

 the base. The pressure increases with the 

 depth in the proportion of the numbers 

 1, 3, 5, 7, 9, &c. — e.g., at 2 feet deep there 

 will be three times the pressure on the 

 sides as at 1 foot, and at 3 feet deep there 

 will be five times the pressure, and so on. 

 From the preceding remarks it is obvious 

 that, as the lower plates," or parts, " of the 

 tank have to sustain so much more pres- 

 sure than the upper ones, these parts 

 should be correspondingly stronger. 



" When a mass of fluid contained in a 

 vessel is in a quiescent state, every particle 

 is pressed in every direction with a force 

 equal to the weight of a column of the fluid, 

 whose base is the particle pressed, and 

 whose altitude is equal to the depth of the 

 particle below the surface : hence the 

 pressure on any particle varies directly as 

 its perpendicular depth beneath the upper 

 surface of the fluid. The lowest parts of 

 a fluid, therefore, sustain the greatest 

 pressure, and they exert perpendicularly 



