EDD

339

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if 8o ae 'ore their leading end got down to the bottom ; andWpi tke y were sufficiently fast: but as they sometimesSchorl to drive more freely than at others, the followingthey n. ' Vaa used to render them fast, for a certainty, whent( ? ' ke bottom. The leading end of every trenailSplif ^ w *' k a saw > for about a couple of inches, and into thishisg j 3 s introduced a wedge, about one-eighth of an inchtJHiift ° re <idth than the diameter of the trenail; it was a full°i' an inch in thickness at the head, and sharpened tob °<% wh<in thsrefore the head of the wedge touched the^°m d °‘the hole, the trenail being forcibly driven thereupon,hisy t , ent er u P on it> till the whole substance was jambed soHihj at „ tke trenail would drive no farther; and as the wood® a Wit' a terwar ds swell in the hole, and fill the little irre-^®6etn 8 kor ing by the jumper, it became so fast, that, ashe dr 3 > they could sooner be pulled in two than the trenails*°° ion \ ° U . t a S a in- The trenail (originally made somewhat*ts up being then cut off even with the top of the stone,itetoraU en< i was wedged cross and cross. There beingPo^gj. / two trenails to each piece of stone, no assignablet&ila j n ess than what would by main stress pull these tre-H f;n s ‘wo, could lift one of these stones from their bedsS was n 6d ’ exclusive of their natural weight, as all agita-thu a g 3 Prevented by the lateral wedges. The stone beingar| <l t} ((! ?’ a Proper quantity of the beat mortar was liquefied,! lrfa Ce ■J, 01nts having been carefully pointed up to the upper5ti 4 ^ ’ he grout so prepared was run in with iron ladles,v °i ( l Sr , S brought to suchh t *> c e; “

‘thi

a consistency as to occupy everyj an d though a considerable part of this was water,

pointed, and, when necessary, defended by

~" al ent ,, ein S absorbed by the dry stones, and the more con-S, tlii 8 rts set tled to the bottom, the vacuity being at ther w-io v T as repeatedly refilled till all remained solid: the% of s , th en ' - - - ' ‘ ~

'«"jij Poster.

« they Severa l courses, represented in Plate III. are shown“cit ^uld a pp earj w hen completed with the whole ofderail “ 6S an< ^ trenails : and besides these, there being also%(!, tj^ Uv ° lewis holes upon the upper surface of each^Coe&i; ° 3e served as temporary fixtures for the work of the

a “ It, 2 cour se.

Jot th ^ the

h °br Se yr«ement was completed and the centre stone ofbv v. Was landed. Of the preceding courses, each was^ ip ' ke stones that engrafted in the dovetail recessesSo as r ° ck > these stones, therefore, being immovableh,. 3l S na ble force acting horizontally, rendered thoseJ fJ|J 8ht tlla t depended upon them ; but having nowat t Vailt an e ' e w bole upon a level, we could not have thishi ll ' f t a° • an y longer ; it therefore became necessary to£. f Po!i e 3 jaiilar advantage by artificial means. For this\ , e u PPer surface of Course VI., ( Plate III.Nth^btd

! same evening’s tide, of the 11th of August,

advantage by artificial means.u Pper surface of Course VI., ( Platec>) ” l hat a b°le of one foot square cut through thew e loot Sa ° ecu P-ed the centre ; and also eight depressions, of%• . tlUp ai " e ’ sun k into that course six inches deep, whichhia^' 68 at ro gular distances round the centre : these

Wer ey oall e( j

!°hr V l° r the reception of eight cubes of marble, in

' "““ed joggles. As a preparation for setting thejStJ? 0ne of Course VII., a parallelepiped (winch, forSt? 8ake . I will call the plug) of strong hard marblelal r ° Cks near Plymouth, of one foot square and twenty-y'Hy ln i engt] wag se t with mortar in the central

N, ‘W

. V® 5 Will* 1 uvitw .- .

therein firmly fixed with thin wedges. Course

>'teen inches in height, this marble plug, which

tL thr “‘ones in neigiu, nus iiuuuio ^“.51

'll. ^titro ° a §b, would rise nine inches above it ; upon this,e **-- - -

li it s (.° lle ( See Plate IV. Course VII.) having a holeh'eticg ®® ntre of a foot square, was introduced upon thetlie plug, and, being bedded in mortar, was in

like manner wedged (with wedges on each side of the plug)and every remaining cavity filled with grout. By this means,no force of the sea, acting horizontally upon the centre stone,less than what was capable of cutting the marble plug in two,was able to move it from its place : and to prevent the stonemore effectually from being lifted, in case its bed of mortarhappened to be destroyed, it was fixed down in the mannerabove described, by four trenails ; which being placed nearto the corners of the large square of that stone, they notonly effectually prevented the stone from lifting, but aided fhecentre plug in preventing the stone from moving angularly,or twisting, which it might otherwise have done, notwith-standing its weight, wjiich was two tons nearly.

“ After setting the first centre stone of Course VII. weimmediately proceeded to set the four stones that surroundit, and which were united thereto by four dovetails, project-ing from the four sides of the centre stone. These stonesbeing fixed in their dovetails by a pair of wedges on eachside, at bottom and top, as has already been mentioned, andheld down by a couple of trenails to each surrounding stone,and still farther steadied by joint wedges at the head of thedovetails, and also in the mitre, or diagonal joints, betweeneach surrounding piece ; the whole formed a circular kind ofstone of ten feet diameter, and above seven tons weight: andwhich being held down by a centre plug and twelve trenails,became in effect one single stone ; whose circumference wassufficient to admit of eight dovetail recesses to be formedtherein, so as to be capable of retaining in their places acircle of eight pieces of stone, of about twelve hundredweight each, in the same manner, and upon the sameprinciple, that the radical pieces of stone were engrafted intothe dovetail recesses of the rock ; and which being in likemanner wedged and trenailed, we proceeded with circulartiers of stone, in the manner shown in Plate IV. Figure 1.It is, however, to be remarked, that the mode of applying thewedges and trenails being sufficiently explained in the seve-ral figures of Plate III. and also in Plate IV. Figure 1, toavoid a repetition of small work, theseveral succeeding figuressimply show the general shapes and disposition of the diffe-rent pieces composing a course, and other incidental largermatters, wholly omitting the particular application of thewedges and trenails ; yet it is to be observed, that they wereeverywhere equally applied, till we got to the top of thesolid.

“My much esteemed master and friend, Mr. Weston, whocame from London to be witness of our proceedings, arrivedat Plymouth during this interval. I went off with him earlyon Wednesday morning, the 17th, attended by Mr. Jessop andhis company, and landed upon the rock at ten : Ilichardsonand company were then about to begin to set the fifth tier,or circle of stones, which was to contain the eight cubesbefore described. These cubes were so disposed upon thesurface of Course VI. that the cavities cut on the under sideof Course VII. to take the upper half of each cube, shouldconstantly fall in the broad part of the stones of the filthcircle ; which will appear plain by considering the dottedlines relative to Course VII. upon the surface of Course VI.(see Plate III. Figure 6.) There could consequently be noapplication of wedges in the upper course, to the fastening ofthe circle of stones, (No. 5.) upon their respective cubes:when therefore the stone respectively came upon them, weput as much mortar upon the top of the cube as would inpart make good the joint between it and its cavity, but notenough quite to fill it; because, if too full, there was noready way for the superfluous mortar to escape ; but a hole,of the size of those for the trenails, being previously boredthrough each of these pieces, answerable to the middle of