706
THEORY AND PRACTICE OF ENGINEERING.
Book II.
the strength of the stone compared to the weight which it supports, is holder than that ofthe columns of the church of Toussaints at Angiers : these piers are constructed in hardstone of Bagneux in the lower part, and in that of Mont Souris in the superior part:the beds formed of the stone of Bagneux are considerably split, those in the stone ofMont Souris have resisted much better. The weight supported by a surface of 9 squareinches in the pillars of the church of St. Genevieve is 16 cwt. ; the weight under which acube of 2 inches in hard stone of Bagneux was crushed is 137 cwt. ; the weights supportedby a similar cube in the stone of Mont Souris was 68 cwt. Hence the pressure supportedby the stone of Bagneux in the piers of St. Genevieve is from eight to nine times lessthan that which is necessary to crush it, and the pressure supported by the stone ofMont Souris is only four times less; it may then appear astonishing that the latter hasresisted better than the preceding.
The pressure supported by the key-stone of the vaults of the bridge at Neuilly is 127$tons per 3 feet 3 inches of length, and it appears that the weight carried is fourteen timesless than that which will crush it: but we must observe that by the settlement in thesevaults, the weight is not equally distributed on the whole height of the key ; the principaleffort is near the upper edge, and although we cannot exactly judge of the total weight car-ried by the adjacent parts of this arris, we may presume, on account of the great flatnessof the vault, that this portion is very considerable, and that the bridge of Neuilly is asbold an edifice in this respect as it is in every other.
We may add from Mons. llondelet an indication of the pressure exercised on a surfaceof 9 square inches in the edifices regarded as the boldest: —
Piers of the dome of St. Peter’s at RomePiers of the dome of St. Paul’s at LondonPiers of the dome of the InvalidesPiers of the dome of St. GenevieveColumns of St. Paul's without the wallsPiers of the tower of the church of St. MeryColumns of the church of Toussaints d’Angers
PoundsAvoirdupois.- 1022$
- 1190
_ c)22
- 1840
- 1235
- 1837$
- 2767$
In the pier of the Chapter House at Elgin the stone supports a weight of 5$ tons on each9 square inches •. it was formerly loaded with a heavy roof covered with lead ; the stone,which is a red grit, has resisted this pressure for several centuries; it is estimated thatthe resistance is from 7725 lbs. avoirdupois to 23925 lbs. on 9 square inches, whilst thatof brick is 9150 lbs. ; consequently*, 1520 lbs. for 9 square inches is a pressure which maybe admitted with security for the voussoirs of an arch. This pressure would be perhapstoo considerable for calcareous stones, and as a general rule, stones should not be made tocarry a weight more than ■fa of that which has crushed them in small experimental cubes;this weight would be too great if we were not assured that the pressure would be equallyspread over the whole surface of the joints : the voussoirs of the arches near the key*, andthe points of fracture, are exposed to inequalities in the partition of the powers they exertas supports, and we shall see bv-and-by how we can appreciate these effects. The partsof a block of stone comprehended between the beds of a quarry are not exactly homo-geneous : experiments have shown that the weight and the specific resistance are more consi-derable in the middle of the stone, and diminish proportionably in its approach to the upperand lower beds.
By the experiments of Gauthev it appears, that the strength of the stone augments in agreater relation than the surface of Us base, the results reported by Mons. Rondelet agresin general with this assertion. But as no experiments were made on surfaces of any con-siderable dimension, or differing much from each other, it does not appear possible to judgeexactly of the manner in which the resistance of the stone does augment relatively to thesurface of its base, and it is proper in the application to suppose the relation constant,which, moreover, cannot induce any dangerous error as this hypothesis, is favourable to theresistance.
We know a little more positively the influence which form has on the resistance of stone.The experiments which have been made with a view to discover this influence have shownthat the different solids, the bases of which had equal area, resisted best, as their figureapproached a circle ; and in general that for figures differing from each other, the resistancewas nearly in the inverse ratio of the perimeter.
When the base of solids remains the same, height influences their strength : a very thinstone easily fractures : if it is in the form of a cube it carries a considerable weight, butif the height is augmented, the strength, which first augmented also, finishes by diminishing;if a vertical prism is divided in several parts longitudinally, it will resist less than if itwere in a single piece.
On the Pressure, to which Stones are exposed in Vaults. — The thickness which it is necessaryto give to the vaults of bridges involves many considerations ; and the pressure to which