a paffage in a lake every where shallow, neceffarily occafioning a whirlpool dangerous to navigation; it feems reafonable to account for the height of the water, not ordinarily exceeding a particular known alti-t tude, in the following manner, Before the autumnal season of the year, when the rains begin to foften the earth, and fwell the rivers, the water difcharged at Toom, is very inconfiderable, so as not to afford a depth greater than that, which may reach to a fhoe buckle, or the knee of a perfon wading; and once it happened, that a perfon taking the opportunity of an in-blowing wind, walked over dry fhod (a). But at the fame time the influx of water is inconfiderable. The upper Ban, which may be fuppofed the greatest of the eight rivers, for it eminently gives a name to all the reft, when they flow in one channel to the fea, being called the lower Ban, has been frequently obferved to have fcarce any current water in it, immediately before the falling of the great rains; and upon this account is not reckoned by fome, writers, amongst the rivers of Ireland, but is confidered as a large brook: The definition of a brook being, a water flowing in a known channel, in the form of a flood, owing to a fudden fall of rain in the neighbouring mountains, whence it takes its rife. The Bleachers upon this By the fame experiment it likewife appears, that the evaporations made in May, June, July, and August, (which are nearly equal) are about three times as much as what evapo. rated in the 4 months of November, December, January and February, which are likewife nearly equal, March and April answering nearly to September and October. The fleece of vapours in ftill weather hanging on the furface of the water, is the occafion of very strange appearances, by the refraction of the faid vapours differing from the common air, whereby every thing appears raised, as houfes like fteeples, fhips as on land above the water, and the land raised, and as it were lifted from the fea, and many times feeming to overhang. And this may give a tolerable account of what I have heard, of feeing the cattle in the Ifle of Dogs, at high water time from Greenwich, when none are to be feen at low water, (which fome have endeavoured to explain, by supposing the Isle of Dogs to have been Hfted by the tide coming under it.) But the vaporous effluvia of water, having a greater degree of refraction, than the common air, may fuffice to bring thefe beams down to the eye, which when the water is retired, and the vapours fubfided with it, paffes and confequently the objects feen at the one time, may be conceived to difappear at the other. (a) This is a broad bank near the Eel waers, over which when the water has paffed, it flows in a much narrower channel, being five and twenty feet deep in part of it, and flow, ing with rapidity. The channel contains two eel waers, one of which is rendered useless by a great quantity of fand thrown into it by the water in ftorms.. this brook, for fuch it may be called, although it be a confiderable flux of water a great part of the year, are well acquainted with the phænomena of it, and do frequently in a fair fun-fhining day, hear the roaring of a torrent at a mile's distance, the mischief of which they can not always prevent, but muft fometimes fuffer the damage of having fifty pieces of linen washed off the green, which, although often found, being stopt by rocks and bushes, are often torn. The courfe of this brook from the fountain head in the mountains near Rathfryland, to the lough in a direct line, is not above four or five and twenty miles; and fhowers in mountains being frequent, when there is no appearance of them in the vallies, moft rivers or brooks whofe courfes are fhort, upon this account are fubject to torrents. The Poet fays, Rufticus expectat dum defluat amnis, at ille Rufticus here does not fignify what is vulgarly thought a countryman or peafant in general,, but particularly a mountaineer, who being well acquainted with the nature of torrents in the mountains, which may Fife and fall, in lefs than the space: of an hour, thinks when he defcends to the vallies, and feeing a regular flowing river, that it is of the fame nature of a montaneous torrent, and that in a little time he may walk over dry. But in this he is deceived; For labetur in omne ovum. At the conclufion therefore of the fummer, fuppofing it a dry feafon, there is very little water flowing into the lough, fince the other feven brooks, or rivers are fingly inferior to the Ban in quantity of water. When the rains fall in abundance, and the brooks or rivers fwell above their banks, and continue fo during five months, and fometimes more, there is a prodigious quantity of water, in fo much that the discharge at Toom being vaftly lefs, the water of the lough usually rifes from fix to nine feet perpendicularly, and fpreads over about ten thoufand acres of land, more than it does when it is at the loweft, which is about 100000 acres. In the fpring of the year when the eight rivers are reduced to rivulets, by the drying winds in March and May; the influx of water is much less than the efflux ; the discharge at Toom is all that time. time very confiderable, and the lough is every day fubfiding, and the flat grounds about it becoming useful for bleaching, pafture, and meadow. The discharge of this great body of water, may be estimated in the following manner. pro The lake is about 60 miles round, which may be reduced to the form of a fquare of fifteen miles a fide, for the fake of computation, although being nearly circular, it does in that form contain a much larger area of furface, than in the fame number of miles in a square from a known perty of the circle, being the moft capacious of figures: Multiply 1760 yards in an Englifh mile, by 15 the fide of the fquare, and then that product into itself, and you have 696960000 fquare yards in the furface of the lake, and fuppofing the lake to be at a medium fix feet deep all through, computing the height of the water from the furface, when it is loweft in fummer, to the furface when it is highest in winter, although the difference for the most part is much greater, except on the lands which are only flooded in winter: Yet at a medium fuppofing the increafed height fix feet, or two yards all over, and multiplying the number of fuperficial fquare yards by 2, you have 1393920000 cubic yards of water; and fuppofing the channel at Toom, the common vifible difcharge of all the water out of the lough, to be only 100 yards broad, and its depth three yards, it being reduced to an equality, although in the deepest part it be five and twenty feet (a), hence the profile of water in this place is 300 fquare yards. This multiplied by 48 miles (or 84480 yards) which the water may be allowed to run in 24 hours at 2 miles in an hour, gives 25344000 cubic yards of water discharged in 04 hours, which dividing 139392000, the cubic yards of water expreffing the whole mafs of increafe in the winter, you have a quotient of 55 exactly, expreffing the number of days in which the whole mafs of additional water, may be difcharged; provided there was no influx at the fame time; but there is a very confiderable one at the beginning of the computation, and it leffens afterwards, till at laft in the drieft time. of the year, it becomes very little: The discharge of this influx of water may be accounted for another way, being carried up in vapour, and may be computed in the following manner. (a) See note (a) page 154. That That one tenth of an inch of the water of the lake, is raised per diem in vapours, may not be an improbable conjecture from the conditions mentioned in the foregoing paper. Upon this. fuppofition every 10 fquare inches of the furface of the water, yield in vapour per diem a cubic inch of water, every space of 4 feet fquare a gallon, and a mile fquare 6914 tuns. And if the lake be estimated as above, as a fquare whofe fide is 15 miles, there will be 125 fquare miles of water, which multiplying 6914 gives a product 8642 50 tuns of water, raifed daily in vapour during the drying. months. This may reafonably be fuppofed, to be a ballance for the influx of eight rivers into the lake, which being fubject to montaneous floods are not at other times very abundant in water. It should be confidered alfo, that the discharge at Toom, upon account of in blowing, or out blowing winds, may flow with a different degree of rapidity and depth of water: For it has happened twice in one century, that the river Rhone at Geneva, where it paffes from the lake of that name, has been stopped by aftorm, where its ufual depth was five and twenty feet, in fo much that ple went down into its channel, and took up fish, medals and other things. Lough Neagh is often interrupted or accelarated in its courfe, in a manner fimilar to this; befides, it fometimes rolls the gravel and throws up a barrier against itself, choaking its own channel, and fooner or later another wind perhaps removes that (a). Hence it happens, that the times of the lakes highest and lowest waters, can not precifely be brought to rules: For they happen fooner or later in the year, according to the agency of all these causes, especially fince the causes which raise thefe vapours from the lake, do alfo at the fame time diminish the rivers during part of the fpring and fummer (b). And in the beginning of the computation peo (a) In the narrow channel where the profile of water is computed page 156. There are two eel waers, one of which is rendered useless by a great quantity of fand, which was thrown in by the water in a storm, making it too fhallow; for the fish run in deep water. (b) If any exceptions are taken to this reafoning upon a fresh water lake, drawn from that already applied to a falt fea; it fhould be obferved, that the difference in weight between falt water and fresh, is not confiderable, and that the fun exhales little or no falt. The Inquirer has observed at the Giant's Caufey, in the North of Ireland, where the fea at high water wathes the tops of the pillars, the fun did evaporate the water, between tide and tide in a hot day, and left in the top of each concave pillar about a Small fpoonful of good falt. Hence should be inferred, that little or no falt is exhaled by the fun from the fea. Mr.. computation when the rivers are full, the lake has by ten thousand acres a larger furface for the fun and winds to act upon, and confequently does then really yield a greater quantity of vapours cæteris paribus. If any perfon knowing the courfe of the waters of the lake, fhould obferve, that at the high rock called the fall at Colerain, which is the only paffage for the waters of the lake to the fea, there be an appearance in the dry months, of much le's water, than paffes at Toom, being the beginning of the channel wherein the water is conveyed to the fea; let it be confidered, that about a mile from Toom, the water spreads again into a small lake of about four miles diameter, as may be feen in Home of the maps prefixt to Lecture IV. particularly that of Speed, from which a great quantity of vapour must arise, and diminish the quantity of water that otherwife would flow to Colerain; add to this the courfe of the water for twenty miles, before it reaches the fall of Colerain, during which space of motion the winds and fun continue to diminish it. Mr. Boyl indeed fays, I have observed, that even the lightest waters will yield a small quantity of common falt. Boyl abridged by Boulton, Vol. I. p. 296. N. B. The Inquirer having made remarks upon the wind, weather and barometer, for a year and a half, in Lurgan, near Lough Neagh, having fixt a weather vane upon a steeple for that purpose, to obferve the direction of the wind, and another on his houfe, to denote the force by means of that and fome machinery within; and having noted down the remarks for every day in distinct columns, he thought to have inferted them here; but the Barometer and one weather vane meeting with accidents, which hindered him to compleat two years obfervations as he intended; (thofe of the time mentioned not being sufficient for his design,) he chufes instead of publishing his own imperfect obfervations, to lay before the reader a fpecimen, of what may be done this way, in imitation of the Gentlemen at Edinburgh, amongst whom there feems to be a spirit for promoting natural knowledge. If fuch obfervations were made with accuracy, in feveral parts of this kingdom, many useful things would attend the knowledge thence redounding. The form of the tables for this purpofe, used by them and the Inquirer, tho' not exactly the fame, might in a great measure anfwer the fame purpose. Meteorological Register. Wind Weather Quantity of rain. Hour Daya. m. p.m. Barom. Therm. 86 S. E. 2 Ours fall fhort of the plan of these Gentlemen, in as much as they had a column for the Hygroscope. See Medical Effays, Edinburgh, Vol. I MDCCXLVII. Such regifters for every County in the kingdom, and for different parts of the same County, where there are mountains or lakes would be extremely useful, with regard to health, gardening, farming, &c.-befides many things of information to philofophers and Speculatifts. |