fequently, the apparent time at the obfervation cannot be known. But if thofe things could be determined by this method, it would still bear no kind of comparison with the other either in point of expedition or accuracy: for the mean change in diftance is almoft four times as great as the mean change in declination, and of courfe the obfervations will admit of an error four times as great; whereas the former will admit of almost four times the accuracy of the latter on another ac-' count, namely, that the limbs of the fun and moon, or that of the moon and a ftar, are fo much better defined than the horizon of the fea is; and it is abundantly proved, by experiment, that plumb lines and levels cannot be used at fea on account of the motion. The prolixity would also be intolerable, especially as perplexed by the number of cafes and varieties which would attend it, according as the declination increafes or decreases, as the altitudes are east or weft of the meridian, or as fome are on one fide and the reft on the other: add to thefe the intricacy of the increments of latitude between the obferv. ations, which will fometimes be north, and fometimes fouth include then the intricacies of parallax, which Mr. Walker has never once taken into the account. For all which reafons, we have not the leaft expectation that this method can ever be brought into any ufeful degree of practice at fea.

A Treatise on Building in Water. By George Semple. Illustrated with 63 Copper-Plates. 410. 18s. I. Taylor.

THIS

HIS work is divided into two principal parts, treating on various fubjects. Each of thefe parts are fubdivided into a great number of chapters and fections, fometimes perhaps rather unneceffarily. Indeed, the work is rather confufedly put together, being a promifcuous affemblage of anecdotes, converfations, directions, journeys, obfèrvations, and extracts from books, and the diaries or journals of the author, who appears to be a plain, well-meaning man, but little accustomed to books, and ftill lefs to writing. However, he does not feem to want good fenfe, nor honesty and industry in his profeffion, which is that of a builder in gene

ral. Of the Effex bridge at Dublin, in particular, which is the principal fubject of the book now before us, he treats in a plain, fimple ftyle,, and in a kind of chronological manner, Mr. Semple delivers the principal matters in the first part of his book; from this it appears that of two of the old ftone bridges at Dublin, viz. Effex-bridge, and Ormond-bridge, the foundations of the piers had barely been laid on the bed of

E e 3

the

the river, which being compofed chiefly of loofe fand and foft mud, the current and floods of water had gradually undermined, and brought fome of them into a ruinous or dangerous condition, infomuch, that of Effex bridge a great part of one of the arches and a pier had fallen down, and interrupted the paffage of carriages over it. The corporation having procured feveral fchemes and proposals with eftimates, for the repair of the broken arch, fome of them with stone, and others with timber, they became the fubject of general converfation; and, in accidental difcourfe, a gentleman afked the opinion of our author, who feems then to have been but a private builder or architect; after a little reflection, he anfwered, that he apprehended there was no difficulty in making an effectual temporary repair with timber, and that he believed it might be done within ten days, for about one hun-' dred guineas,' This fum being but one fifth, and the time one fifteenth of those of any of the propofals or estimates, the converfation was mentioned to the magiftrates of the city, who prefently after requested Mr. Semple, to undertake the repair, which at length he complied with, though reluctantly, and performed the work in the time and for the fum he had at first mentioned, and that in a manner fo much to the fatiffaction of his employers, that they foon requested him to undertake the erection of a new ftone bridge.inftead of the old This propofal at first startled him, as he was but little acquainted, either from theory or practice, with works of that kind, especially the laying of fuch foundations and building in water; on which account he was unwilling to engage in it, and the more fo, as he was then pretty fully employed in the building of feveral houfes. However, being much urged and encouraged to this undertaking, he at length complied with the requifition, and promised to build them a bridge in two years, for twenty thousand pounds, that should last as long as the little adjacent mountain called Sugar-loaf-bill.' And what is rather uncommon, he fulfilled this promife, having built the bridge in the moft fubftantial manner, for within a very few hundred pounds of his eftimate, and opened it within a few days of the time promifed. As he had the most part of his business to learn, he experienced great trouble and delays from that as well as from the natural difficulties attending the fituation and circumftances. However, with great application and induftry he furmounted them all. And as he has pretty fully narrated all his proceedings, both the fuccefsful cafes and those which failed, and particularly defcribed the feveral methods used in the whole procefs, the book may afford many ufeful leffons to young artifts in this branch of architec

one.

ture,

tare, who will alfo learn fome things from the cautions and advice which the author gives in fome of the observations, which he has made.

Mr. Semple's unaffected manner of relating his difficulties and endeavours, &c. is not unentertaining.

I had on many occafions before, fays he, felt a great defire to acquire knowledge in difficult matters of art and fcience; and whenever they grew eafy and familiar to me, I could no longer perceive in them the charms which had before captivated me. Intereft alone could never fway me in thefe refearches and purfuits; but from my earliest days I had entertained a notion, that the greater the difficulty, the fweeter the conqueft. In fhort, that fubject took firm poffeffion of me; and I applied myself to fearch my books, of which I then had a fine and valuable collection, and I found in them numerous and elegant dedefigns; but as touching the laying a foundation in deep and rapid rivers, all the authors were in a manner filent.

This deficiency in fo many excellent authors, greatly amazed me; and in hopes of procuring better inftructions, I fettled my domeftic affairs, and went to London, where I procured about 401. worth of books, plans, &c. which I ordered to be fent after me, and returned home within the compafs of fourteen days, full of hopes that, by their affiftance, I should be able to pick out fomething for my purpofe; but after I had attentively perufed them, I found myfelf in the fame fituation I had been in before.

I cannot defcribe the indignation and forrow I felt, at finding an art of fuch public utility, as that of building bridges confeffedly is, fo fhamefully neglected: however, these bookst furnished me with many afeful hints, and observations, fome few of which, I fhall give for the improvement of the young ftudent, and as neceffarily connected with the fubject I am treating'

And again:

In this dilreffed fituation I had frequent recourfe to my books, all of which could not afford me any fort of encouragement, faving what I have already laid before you: they told me, indeed, to make an inclofure; and fo might they tell a man, that to measure time, he must make a clock; but what would that avail to a man that had never made nor feen any kind of machine for that purpofe?-However, I proceeded to form my plan of the bridge, in projecting of which, I found myfelf most plentifully ftored with precedents, and yet after all (in juftice to Mr. Labelye) I must declare that Westminster bridge deferved the preference; therefore (fave only in fome particulars) I took it for my precedent.'

However, our author foon found it neceffary again to vifit England for farther information, from whence he returned to

Dublin much difappointed; but afterwards entertained the moft fanguine hopes of fuccefs from perufing the 4th volume of the Hydraulic Architecture, by Colonel Belidor.

In digging the foundation of one of the piers; a very curious accident happened, by which it was difcovered that in the thin interstice between a ftratum of clay or loom, which extended under the whole bed of the river, and the folid ftratum of rock immediately beneath it, the fea-water found a paffage without communicating with the waters of the river. This fingular discovery we cannot avoid extracting in the author's own words, and hope the curiofity will be an excufe for the length of the extract.

6

Having got fome part of the north end of the pit funk down to about ten feet beneath low-water mark, and quite free from water, except fome fmall quantity that foaked from the bed of the river, which was then about the fame space above us, and that we conveyed into the S. E. corner for the fcrewpump; the men that were finking for the land abutment perceived one spot in the ground that grew very wet, but did not much regard it; but in finking the next fpade which was about a foot deeper, when they had laid open that wet fpot, we were all greatly alarmed by the very ftrong boiling up of the water just in that place. I immediately called for one of the boringpipes, and had it drove down to the rock, and the top of it was feet above that floor, on driving of which the boiling entirely ftopped. This gave us fome fpirits, and the men went on with their finking that floor, but at this time the tide was at ebb; but when the next tide rofe to about fix feet, it began to boil over the head of the pile, and increased in force as the tide rofe, and fo greatly wet the work, that we found it adviseable to plug up the pipe, and fo we continued to hurry on the work, without taking any further notice of the boiling, except looking on our jetteau as a matter of curiofity, which during the time of high-water and after it, when we would pull out the plug, would play upwards of a foot above the head of the pipe with great force.

The rev. Dr. Hudfon, who was a very curious and conftant obferver of our proceedings, came to the work the next morning, and I brought him with feveral other gentlemen to the work, to fee our curious jetteau: he called for a glafs, and they all tafted, fmelled and attentively obferved its colour. They then dropped a piece of filver into the glafs, which was foon turned to a dark yellowish colour; and at length, they all concluded that it was a mineral fpa, and advised me to fend directly for Dr. Rutty, for whom I went myself, and he cheerfully came with me, the gentlemen waiting for us: he tried all the before-mentioned experiments over again, and acknowledged, that it feemed very like the water of Swaddling-bar; but con

cluded,

cluded, that any fea-water that partook of putrid water, running from a foul fewer, might have the fame effects that had, and he defired me to fend fome of it home with him, which I accordingly did.

This affair engroffed my whole attention, and put me upon making the following experiment. I then had two other fimilar pipes drove at ten feet diftance from it to the eaf and weft, and I bored them in the like manner at low-water, and as the tide came in, I found the water rife in all the three pipes exactly alike, and at or foon after high-water, they all played together when we took out the plugs, near a foot above the heads of the pipes, which were all on a level; and as the tide fell, they abated in their force, but never funk lower than the tops of the pipes, which as I faid above, was 3 feet higher than the bottom of the pit, and confequently 7 feet under low-water mark.

• The next morning Dr Rutty, and Dr. Hudfon came to the work, where they met the gentlemen that were there the day before, and he, Dr. Rutty, then affured us, that it was no fpecies of fpa-water, but he believed, a large body of fubterraneous water, that ran along on the furface of the rock, and communicated with the fea-water, and partook also, of the foul waters of the bed of the river and the fewers. He then produced us as much falt as would cover a fhilling, which he faid, he had extracted from one pint of that water, which was not near so much as a pint of fea-water would produce; and concluded with giving me this friendly and ufeful caution, "take great care that you do not let that water break up upon you, for if you do, you will never conquer it."

This opinion of Dr. Rutty's coroborating with my own, it was inftantly circulated among all the men in the work. They all unanimously joined in exerting themfelves to the utmost of their abilities; for that water alone, was not the caufe of our anxiety, as we were at that time thirty-one feet deep under the furface of the pavement, which was within ten feet of being perpendicular over us, and all being made ground, with a fine fea fandy bottom, which notwithstanding all the precautions we had taken in finking and fhoring up our breaft-work, we well knew, that if the malleft quantity of water fhould then get into the pit, it would most affuredly fap and foften that fine fand, which together with the continual making of the ground by the carriages, would render it impoffible for us to prevent its burfting in upon us, and in all probability pull in the corner house along with it.

Hereupon we agreed, that it would be best and fafeft for us to defift from finking any deeper, though that was no small difappointment to us, as we then had thoughts of going down to, or very near the rock, nor otherwife had we any difficulty to encounter with in accomplishing it; for the floor we were then leyelling and clearing out next to the bank, was perfectly free from

water,