large cascade over part of it, during the rains. A sinall pagoda is lately built over this rock. The other natural curiosity is a flaming well among the hills, about 4 miles to the southward of the rock, that blazes on the surface of the water, which the people of the country have inclosed with brick-work, in the form of a funnel or chimney, that draws the flame to a point, and makes it burn fiercer. The flame issues also with the water through some holes left in the brick inclosure, for conveying it to an adjacent cistern, like fire confined, and wanting vent. The water thus let out bubbles like a boiling pot, but close to the flame is only lukewarm. A pagoda, built also over the well, is in a constant misty fume, much like what rises from the waters at Bath in England, the taste of which, he says, is like this, having drank of both. LXV. On the Extraordinary Agitation of the Waters in Mount's Bay, and other Places, March 31, 1761. By the Rev. W. Borlase, M. A., F. R. S. F.R.S. p. 418. March 31, 1761, about 5 in the afternoon, there was an uncommon motion of the tide in Mount's-bay, Cornwall. It was full sea that day about half an hour after 12. After the tide had ebbed about 4 hours and half, instead of continuing to retreat gradually, as usual, till it had completed the 6 hours ebb, on a sudden it advanced as it is usually at the time of the moon, at an hour and half before high water. It then retreated near to the point of low water, then it advanced again, and retreated, making 5 advances, and as many recesses, in the space of one hour; viz. from about 5 to 6 o'clock; which was the whole time that these uncommon stretches of the tide continued. But the first motion was most considerable, the sea advancing the first time to a quarter ebb; whereas the 2d advance was but as far as the sea reaches at half ebb. At the first surge the waters rose at this place 6 feet perpendicular. At the pier of St. Michael's mount, 3 miles to the east of Penzance, the tide was observed, at the same time, to rise and fall about 4 feet. At Newlyn, a mile west of Penzance, the tide rose to the same height nearly as at Penzance. At Moushole pier, 3 miles s.w. of Penzance, it was only observed that the sea was in great agitation, and the fishing-boats in danger. At the islands of Scilly, the sea was judged to rise about 4 feet; but the agitation continued longer than in Mount's bay, viz. more than 2 hours. On the coast of Scotland, from Fort Augustus on Lochness, we had accounts that on the same 31st of March, about 2 in the afternoon, Lochness rose on a sudden about 2 feet perpendicularly, and continued alternately rising and falling for the space of of an hour. The king's galley broke from her moorings, and VOL. XI. drove into the loch; several boats were cast very far upon dry land; in the middle. of the loch the water swelled up like a mountain, extremely muddy, and the motion was attended with a very uncommon hollow sound. By a subsequent account from the same place, the water is said to have risen near 30 inches, between the hours of 12 and 1 P. M. and continued for near half an hour. It is ádded, an instance similar to this happened here at the time of the Lisbon earthquake in 1755. On the coast of Ireland, from Cork, there was advice that on the same 31st of March, a quarter after noon, a shock of an earthquake was felt in that city, and between the gates only, allowed to be more violent than that of November 1, 1755. It did not continue above one minute, undulating from east to west, and vice versa. At Kinsale, about 6 o'clock. P. M. near dead low water, the tide rose suddenly on the strand, about 2 feet higher than it was, and went out again in the space of 4 minutes, with great force, which repeated several times; but the first was the greatest. At Amsterdam the branches in the synagogue were observed to vibrate between 1 and 2 o'clock. In the great church at Maesland-Slys, the branches moved about a foot from the perpendicular, and the vessels in the harbour were agitated. But this earthquake was felt more violently on the ocean, between the coasts of Spain and the British channels. Ships on their passage from that part of the continent, many leagues to the westward of Cape Finisterre, felt an unusual. agitation of the sea, as if they had struck on sunken rocks; the time agreeing. with that of Cork and Fort Augustus. Captain Woodward, of the Expedition packet-boat, sailed from Lisbon March 29. On the 31st, soon after he had passed the rocks of Lisbon, in the morning, and almost calm, the sea swelled to a great degree, with a rumbling noise. The vessel was tossed about as if in a storm. The agitation continued 4 minutes. The Gosport man of war, off the rock of Lisbon, at past 11 in the forenoon, felt two violent shocks of an earthquake; the first continued near a minute and half, the second not so long. Under the convoy of the Gosport, were several ships, all affected in the same manner. One off Lisbon felt the shock, attended with a noise, as if empty casks had been tossed about in the hold. The ship immediately made a good many inches of water, which proceeded from the seams opening; the vessel shaking in so dreadful a manner, that under the apprehensions that the ship was sinking, the people threw out their boat, and got all things ready for leaving her, then about 50 leagues from shore. In the latitude 43, not many leagues off shore, in her passage from Lisbon, the Amey of Bristol, Capt. Condon, felt a most violent shock. The concussion was so great, that it shook the needle off the spindle of the compass;, and immediately after arose such a storm of wind and rain as he never before met with. The shock was felt 10 minutes A. M. viz. half an hour before it was felt at Cork, and 5 hours before the waters rose at Kinsale, and in Mount's bay on the same day. At the same time there was a violent earthquake at Lisbon, thought by some as severe as that of 1753, but the agitation more equable; consequently the damages not so deplorable, no lives lost, a few old houses shattered and thrown down, and some new ones cracked; the shock lasted between 3 and 5 minutes. But more particular is the account following, from an eye witness, in an English vessel then off Lisbon, i. e. lying before the city. On March 31st, at mid-day a severe shock, not so strong as that of 1755, but of longer duration. I saw the ruins of the last earthquake falling heap upon heap, and turning round beheld the rocks on the opposite side falling from the mountains, followed by a continual cry of the people; the buildings erected since 1755 damaged to the amount of 20,000 moidores at least. It lasted about 5 minutes, some say 7; the water in continual agitation all the afternoon, ebbing and flowing 3 or 4 feet in a very short time. At 12 at night, another shock, of short continuance; that night 3 more; did no damage. St. Ubes, 10 leagues distance to the south, has suffered much; and the villages to the north, as also a large convent. During the confusion in the city, 300 persons in the several jails gained their liberty; but all except 14 were secured again. The shock felt at Oporto was very strong; but did no damage. At a village about 20 miles distant, 3 or 4 houses were thrown down, by which several people were killed.' It is said that the government of Portugal interposed; and to prevent the consequences of terror and fancy, (oftentimes as pernicious as realities) prohibited particular accounts, and even public thanksgivings. At Madrid the violent shock lasted 24 minutes, shaking the houses, and throwing down the furniture; the inhabitants left their houses for fear of being buried in the ruins. It was remarked here as something extraordinary, that at the moment it was felt the atmosphere was quite calm, and a gentle shower fell. At Madeira the shock was felt very violent at 10 o'clock A.M. It did no damage in the town; some rocks were split, and fell into the sea, and some of the roads of the island suffered. The greatest damage there sustained was the loss of one church, and 4 people killed, 2 of which were in a boat fishing near the shore, when the rocks fell. At Barbadoes, about half past 4 P.M. near low water, the sea suddenly retired from the shore, and in about 3 minutes returned again, to the height of near 4 feet. This flux and reflux abated about 8 o'clock, so as scarcely to be perceived; but about 10 it increased again for some short time, then decreased till 6 the next morning. The flux and reflux was not always regular after the first 3 hours; it was sometimes performed in 3 minutes, and at other times in not less than 6. On November 1, 1755, the day the earthquake happened at Lisbon, there was then here, in this same manner, an agitation of the water.' At Fyal island, one of the Azores, and Terceira, the sea rose to a great height, and fell again so low that the quays were left dry; all the lighters and fishing-boats that were hauled up in Portorico, were carried down into the bay, and broken to pieces on the rocks. About a fortnight after, several earthquakes (successively more and more violent) ended not, till on the 20th 3 volcanos threw out as many rivers of lava, of near a mile in breadth, and 4 yards high, which threatened desolation to the whole country, and continued over-running every tree and house till the 24th. From these accounts the violence of this earthquake was greatest at, or rather near Lisbon; perhaps at sea, in latitude between 43 and 44, and longitude about 11° 19′, where no tremors of land could be observed, and consequently the effects not so terrifying, nor perceived by so many, nor so destructive as if it had happened on the land, and contiguous to Lisbon, as that of 1755. The weather various in the different places, but mostly calm. There was a great conformity between the effects of the earthquake of Nov. 1, 1755, and of this of March 31st, 1761; viz. in the extent; in the rise of the waters; in the calmness of the weather in most parts; and in the succession of time, beginning sooner at Lisbon than on the northern shores both times. LXVI. Observations on a Clock of Mr. John Shelton, made at St. Helena. the Rev. Nevil Maskelyne, M. A., F. R. S. p. 434. By The ancients were well acquainted with the rotundity of the earth, and were satisfied, that heavy bodies, in every place, had a tendency to its centre: but they had never any suspicion that the force of their tendency to the centre was greater in one country than another, or that when dropped from any height they fell faster in one latitude than another. The great Huygens, who first set the doctrine of centrifugal forces in a clear light, saw plainly that the weight of bodies must naturally be less at the equator than at the poles; their great velocity there round the earth's axis taking off part of the weight which they acquire by their gravitation towards the earth's centre, And though he was not quite exact in settling the true proportion of the force of gravity in different latitudes; yet we owe this obligation to him, of having made the first discovery of a thing, which has since been the ground of so many theories and experiments. Mr. Richer, when he went to the island of Cayenne, made the first experimental proof of the decrease of gravity, in approaching the equator, though he was not led to it by Huygen's theory, which was then but fately published, and not so generally known; but from finding his clock, which he had brought with him, go considerably slower than it had gone in France. But Sir Isaac Newton first of all showed, how the variation of gravity in different latitudes depended, not only on the centrifugal force, but also on the figure of the earth, which he likewise determined, as well as the proportion of the force of gravity in different latitudes, as far as theory alone could limit them. Mr. Colin Campbell made a very curious experiment, of the diminution of gravity from London to Jamaica, by means of an excellent clock, made by Mr. Graham, an account of which is given by Dr. Bradley in N° 432 of the Philos. Trans., to which he has added a table of his own, expressing the proportion of the force of gravity in different latitudes, and has subjoined the proportion of the equatorial to the polar diameter of the earth, which should follow from the experiment, according to Sir Isaac Newton's principles. A like experiment was made by the learned French astronomers, who went to the polar circle, to measure the length of a degree of the meridian, by a clock made by the same excellent artist, expressly for the same purpose. We are likewise obliged to the gentlemen of the R. A. S. at Paris, for several experiments made by them, in order to determine the force of gravity in different places, by measuring the length of the second pendulum. Mr. M. could not fail of being desirous of improving the opportunity, which his voyage to St. Helena afforded, of examining this curious point, among other experiments: and the R.S. were pleased to furnish him with an excellent clock, with a gridiron pendulum adapted to it, executed by that diligent and ingenious artist Mr. John Shelton, for that purpose. Dr. Bradley set the clock up at the Royal Observatory at Greenwich, and there examined its going, where he found that it lost 11 seconds per day on sidereal time, the thermometer of Fahrenheit's construction, which was placed withinside of the clock-case, standing about 50 degrees, at a medium, during the time in which the experiment was made. Soon after his arrival at St. Helena he set up the clock in the valley near James's Fort, in a place elevated 85 feet above the level of the sea; and by a proper meridian mark, and a transit instrument, he observed the transits in the annexed table; where the first column expresses the day of the month of the observation; the 2d the time shown by the clock, at the instant of the transit of the sun's centre, across the middle vertical wire of the instrument on that day; the 3d column is made from the 2d, by applying the equation of time to it. Therefore the dif- 1761. Time by the Time by the clock, with the equation of time applied. 0h 11m 30 Oh 13m 47* 17 48 21 45.1 27 15 10 29 18 +9 30 20 401 23 44.8 May 3 26 161 29 42.1 6 31 56 35 39 35 45 39 36+ |
