ON ARCHITECTURE AND CARPENTRV. 159 



considerable. But in the case of a building of stone, the strength often de- 

 pends as much on the weight of the materials as on their cohesive power : 

 and the lateral adhesion, which is materially influenced by the weight, 

 constitutes a very important part of the strength. For resisting a force 

 which tends to overset the building, the form in which the weight gives 

 the greatest strength is that of a conoid, or a solid of whicli the outline is a 

 parabola, concave towards the axis: and for procuring, by means of the 

 weight, a lateral adhesion which is every where proportional to the force, the 

 form must be cylindrical. So that in a building circumstanced as we have sup- 

 posed the pillar to be, there appears to be no reason for making either portion 

 of the outline taken separately, convex towards the axis, although the angu- 

 lar junction of the two portions of cones might very properly be rounded off; 

 and the upper parts might be a little enlarged if it were desirable to reduce 

 the thickness of the walls. But the Eddystone light house is completely 

 above the level of the sea, although in stormy weather every part of it is ex- 

 posed to the action of the waves, fhe water being sometimes thrown up to a 

 much greater height than that of the light house: so that it may be consider- 

 ed as exposed to the force of a fluid more and more powerful as it is nearer to 

 the foundation; and in this point of view its form differs but little from that 

 which the most accurate theory would point out; but it is probably a little 

 weaker about the middle of its height, or somewhat lower, than in any other 

 part. (Plate XI. Fig. 150.) 



A wall must be reduced in thickness as it rises, for the same reason as a 

 column is diminishecl; and if the wall is a part of a house, it must be reduced- 

 in a still greater degree, since the load, which is to be supported by it at dif- 

 ferent parts of its height, is usually much varied, by the weight of the floors, 

 and of the contents of the apartments. But sometimes the obliquity of the 

 surface of the wall may become inconvenient, by promoting the growth of moss 

 and weeds. In building a wall, the first precaution that is required, is to 

 dig deep enough to ascertain the nature of the ground; the next, to lay a 

 sufficiently extensive and firm foundation ; and it has been very properly re- 

 commended that where a well is wanted, it should be dug before the founda- 

 tions of the house are laid, in order to examine the qualities of the different 

 strata which are to support them. The disposition of the stones, or bricks, is 

 not a matter of indifference ; the strength is obviously greatest when all the 



