be fufficient for the expediting the work; but the latter will not be wanted till we begin to take off the water.

With the bleffing of God, if I continue in good health, I propofe to be at Perth in fix weeks time, from the fetting off on my journey to my coming again, provided all materials, or part to begin, be got to a proper place to work, upon notice, which I prefume will be fent; and I think, if agreeable to you, Gentlemen, that Mr. SANDEMAN's faw-mill will be the propereft place to work up the materials, as the stuff may be fawed by the mill and worked up there, and then fent down to the work by water as wanted, and not be liable to be any of it loft by floods.

Which report being read, the Commiffioners direct Mr. GWIN that he will imme→ diately, upon his return to England, communicate the faid report to Mr. SMEATON, and defire Mr. SMEATON to ascertain the materials which he shall think moft proper and necessary for the timber work that is to be prepared for erecting the experimental pier; and that he would, as foon as poffible, tranfmit to Mr. PATRICK MILLER an account of the materials so ascertained by him.

The Commiffioners approve of Mr. GWIN's terms upon which he proposes to bring down from England the two workmen and his apprentice.

The Commiffioners recommend it to Mr. GwIN to regulate his return fo that the neceffary preparations may be ready at the time it will fuit Mr. SMEATON'S convenience to be here; and they direct their clerk to deliver to Mr. Gwin a copy of their minutes in fo far as relates to his report.

Extracted from the records of the faid Commiffioners by me

DESCRIPTION and METHOD of fixing the foundation of the fecond pier of Perth Bridge, according to the plan.

of

Method of fixing the coffer-dam.

THE gravel turning out harder than was expected in the last pier, and it taking up much time in driving the piles of the coffer-dam down to their proper depth, and alfo finding them very difficult to draw, and much shattered when drawn, I propofe for this pier that as many additional piles be procured as will fet the whole at the distance 9 feet from fheeting of the base of the pier, and to drive them no farther than to fix them firm in the ground, which if that happens at 2 feet will be fufficient. The dam being then compleated to its proper height, in order to guard against filteration of the water under the bottom of the piles, I propose to throw in all round on the outfide a quantity of gravel and corn mold earth mixed together, fo as to lay rather floping against the piles, and extending about 6 feet all round: the gravel being mixed with the earth, will not only augment its quantity, but prevent its being carried away by the ftream, the use of the earth being to choak up the chinks and pores of the gravel upon the bed of the river. I apprehend about a cube yard of compound matter, to a yard running, thrown in, will be fufficient, obferving to begin the work at the falient angle up ftream, and proceeding gradually downwards on both fides, clofing at the falient angle down ftream..

Method of making the excavation.

The pumps being fixed, and the water pumped out, begin the excavation no larger than the base of the pier, and having got down a fpace in the middle to its proper depth, increase it in width and length till the area is clear for driving the piles upon which the foundation frame is intended to reft, and no more, leaving the matter on the outside of that area to form its own flope toward the coffer-dam, fo that the reft of the area will be left folid, to fupport the sheeting of the dam; and if any part seems feeble, or likely to give way, let it be ftrengthened by driving piles for fupporting flate on edge, as the nature of the ground and circumstances shall fhew to be neceffary, obferving, in beginning the excavation, that the matter be thrown out all round, fo as to fecure and strengthen the matter firft thrown on the outfide of the dam.

The depth of the excavation will be found by the following rule. It must at least be excavated 3 feet at a medium below the natural furface of the gravel where the pier.

ftands;

ftands; but if this does not carry down the bafe of this pier within 2 feet of the level, at which the base of the firft pier was fixed, let the depth of the excavation be increased till it is within 2 feet of the former depth.

Method of fixing the foundation, according to the plan.

The excavation being made, as far as is above directed, let the 21 piles, upon which the frame refts, be driven into their proper places: thefe piles are to be 10 inches heads, and of 6 feet long, fuppofing the gravel of equal ftrength with the laft, but if there is any reason to suppose it stronger or weaker, the length above mentioned should be increased or diminished. This being done, and the pile heads reduced to a level, lay down the frame thereupon, which I fuppofe to be ready prepared in the yard, with the tye beams ready fitted with dovetails thereto, and being trenailed down upon the refpective pile heads, proceed to drive the fheeting piles, which may be of oak, elm, beech,. or fir, as can best be got. I fuppofe them to be of 6 feet long; they may be driven plain,. as fhewn on one fide, as was done at Coldstream Bridge, but would be preferable if rebated, as fhewn on the other half. The choice of the method depends on circumstances; if, from experience of the other pier, they are like to drive regular, without tearing of the rebates, by meeting with great ftones, &c. then they will both drive more regular, and hold firmest by being rebated, but if they are apt to fplit, then it will be as well to fave that time and labour, by making them plain, and more especially fo if the dif ference of expence in workmanship, or hindrance of time in doing it, is likely to be a material object, in that cafe the rebating may be omitted.

N. B. If driven plain, the breadths of the piles are not material; but if rebated, narrow piles will enhance the workmanship in preparing. In order to fave timber (and especially if fir piles are made use of, it will be much stronger) I propose to groove the piles on both fides, and to nail in the tongue, which, if fir piles, may be of harder wood; the best proportion for fir piles would be to make the tongue 1 inch thick, and 12 broad, to be let into the fide where it faftens, and to stand out 1 inch; but this may be done according as the tools already prepared may fuit, there being no need to make: new ones on purpose.

I

The tops of the sheeting piles being reduced to a level with the ftring pieces (and fpiked thereto as they are driven) the outfide must be reduced to a regular breadth, fo as to take the notched ftones in a line. This being done, the reft of the bearing piles must be driven, beginning with the outfide rows, and cut to a level with the top of the

ftring pieces; thefe piles may be of 6 feet, more or lefs, according as the others are found to go.

The fetting must be compleated by firft underpinning the ftring pieces and tye beams, as firmly and equally as they can, by moderately driving ftones under them; and lastly, the other spaces to be fet, and well drove down as before; but before they are rammed down, the joints of the fetting fhould be filled by fweeping in dry lime mixed with fand and small gravel, that when drove down the whole may be compact together.

When the pier is got above low water, I would have the matter taken out for 4 feet wide round the pier, down to the level of the top of the notch course, and filled with good lagging as before, ftanding fomewhat higher than the natural bed of the river, and the rest of the space covered with rubble to the fides of the dam.

N. B. I suppose the bearing piles to be fufficiently drove, when it takes 20 blows of a fufficient ram to drive an inch, and the sheeting piling to be fo when it takes 40 blows to drive the fame quantity; but the sheeting piles should be drove as near to a regular depth as poffible.

Aufthorpe, April 23, 1767.

J. SMEATON.

P. S. I don't mean to fet afide the use of such materials as were prepared according to the plan of last year, unless utterly inconsistent with the present.

PERTH BRIDGE.

Perth, October 5, 1787.

HAVING this day viewed the bridge of Perth, I have the fatisfaction to obferve every

thing relative to it in perfect good condition, the walking path over it excepted; which, from the want of hardness in the stone wherewith it has been laid, is not only much worn, but from the hard pebbles imbodied in the stone itself is become very rough and uneven. For the effectual restoration of this part, nothing would be fo compleatly durable as to remove the present stone-work, and relay the fame with Aberdeen granite of the fame depth as the prefent. But as the prefent ftone-work of the walking path appears

to

to me to lay compleatly firm and folid, and being fhewn a fample of the Kingudie flat paving, which I understand can be procured of any thickness, with parallel furfaces, and alfo appearing to me to be of a nature fufficiently durable, I apprehend the work may be executed in a fubftantial manner, as follows:

The stone from Kingudie being procured of 4, 5, or 6 inches thickness, aз can be most conveniently had, and of fuch lengths as to make good the whole breadth of the walking path without a joint, but of promifcuous breadths in the direction of the bridge, I would advife to chiffel or broach off the upper furface of the prefent walking path till it comes to a regular height, and then with good mortar to lay down the Kingudie flooring, clofe jointed, upon it, making the joints of the flooring with the best Pozzelana mortar ; and in cafe there is Pozzelana remaining in plenty, it would be well to give the mortar wherewith the flooring is bedded an allowance of Pozzelana, as fuppofe, half the quantity; obferving to level the furface of the present mass of stone in fuch manner as to allow a drip or declivity from the parapet towards the carriage way of about 3 of an inch; and alfo to make the breadth of the new flooring about of an inch fhort of the prefent breadth, fo that the tread of the carriages being taken off from rubbing against the Kingudie flooring, it may remain undisturbed being defended by the folid of the present; and in the fame way the walking path may be fet with Aberdeen granite, provided it is worked to a parallel thickness.

With refpect to the paving of the bridge, there is no objection to it as to the ftructure, this being merely a matter of convenience, of which those who from their local fituations observe and use it can be the best judges. I have only to obferve, that the Aberdeen granite paving is not only the moft durable of any that I am acquainted with; but, from its roughness, the horfes feet are the most steady, and hold the firmeft in drawing upon it. The blue whin is the next in degree; fo that where the blue whin is procured upon the place, I cannot think it neceffary to fend for the Aberdeen granite from a distance.

Having viewed the bulwarks that have been erected oppofite the Inch upon the northeast side of the river, I am of opinion that all fuch works as cross and interfect the stream of the river, or interrupt the water from gliding freely away, are the means of increasing the stress upon other parts, and therefore, as affecting other properties, should be avoided. When properties want a defence against the incroachments of the waters, it should be by difpofing those defences at the foot of the banks, in a direction parallel to the stream, and without interrupting its free courfe..

J. SMEATON,

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