A Letter from Mr. Halley of June the 7th. 97. Concerning the Torricellian Experiment Tryed on the Top of Snowdon-Hill and the Success of It

IX. A Letter from Mr. Halley of June the 7th. 97. concerning the Torricellian Experiment tried on the top of Snowdon-hill and the success of it. LAST Whitson-week by the favour of Mr. Davis, a good Friend of Dr. Trancred Robinson's, I had the opportunity of seeing what I pleased in North Wales, and was on Wednesday May 26, on the top of Snowdon, where I tried the Torricellian Experiment with all the satisfaction I could wish for; the Air continued both before and after in the same state as I got it verified by Mr. Davis's standing Barometer at Llanerch in Denbighshire, about 25 Miles East from Snowdon where it was Observed as follows. | Date | Time | Barometer | |------|------|-----------| | May 25 | 8 15 | 29 7 | | | 10 8 | 29 8½ | | 26 | 8 16 Morn. | 29 8 | | | 11 00 Morn. | 29 8 | | | 3 00 Aft. | 29 7¼ | | | 6 00 | 29 7½ | | | 9 30 | 29 7½ | | 27 | 6 00 Morn. | 29 7 | | | 10 00 | 29 7 | | | 3 30 Aft. | 29 7¼ | | | 6 15 Aft. | 29 7¼ | | 28 | 5 00 Aft. | 29 8 | | | 10 00 | 29 8 | Llanerch is about a Mile and an half above the Town of St. Asaph, about 6 Mile from the Mouth of the River River Lluyd, which falls with a rapid Stream into the Irish Sea; and consequently, is several Foot above the Surface thereof. May 26. Between One and Two in the Afternoon, on the top of Snowdon, I thrice repeated the Experiment, and as often found the height of the Mercury 26 Inches 1. And being come down to Llanberis, at the Foot of the Hill, about Six that Evening, I as often found it 29 Inches 4. A little above this place, are the principal Fountains of the River, that falls into the Channel of Anglesey, at Carnarvon, call'd anciently Segontium, whither we went the next Day; and about Eight at Even, found the Mercury, by a triple Experiment, to stand at 29 Inches 9°, very near the Surface of the Sea: When, at the same time, at Llanerch, it was not above 29 7½; whence I conclude, that the Difference of the Airs Pressure on the Sea, and on the top of Snowdon, is rather more than 3 Inches, 8 Tents. I could have wish'd for one of Mr. Hunt's portable Barometers; which will certainly be accurate enough for taking the Levels, for bringing of Water from distant places, and certainly much less subject to Error; there being a Tenth of an Inch for each Thirty Yards, which may be divided into many parts evidently. Snowdon was measured by Mr. Cawwel, with Adams's Instruments, to be 1240 Yards high; which abating the height of the Mercury 3 Inches, 8 Tents, may serve for a Standard, 'till a better be obtained on a higher place. From hence the Sea dipt every-where above a Degree below us, the Visible Horizon being a lesser Circle, and we saw Ireland plainly, from the WbS to SWbW, and then appearing in the NNW, and the Mountains of Cumberland or Westmoreland very fairly, but evidently in the North; and I think we saw as far as St. David's Head into the South; Carnarvonshire and Anglesey lay under us, like a Map, affording a very pleasant Prospect, were it not for the Horrors of the Neighbouring Precipices. Hence we counted fifteen or sixteen Lakes, great and small, where the Cavities of the Rocks are filled up with the Rills that gleet from the Hills; all these are said to abound with Trout, some of which we found to be especial good Fish: And in one of these Lakes I was on board a floating Island, as it may be called; the Lake is scarce half a Mile about, environed with a boggy turfey Soil, a piece of which, about six Yards long and four broad, floats on the Water, being about five or six inches raised above it, but it, I believe, about eighteen Inches deep within the Water, having broad spreading fungous Roots on its sides, the lightness of which Buoys it up. It was driven on the Lee Shore, but I lanched it off and swam it, to be satisfied it floated: This I take the more notice of, because it is denied to be true, by the Author of the Additions to Cambden, lately Published: But I myself saw it as described, and was told it had formerly been bigger, there being a lesser spot, that they told us had been heretofore a part thereof, which floated likewise. I have not time nor Paper to describe this horrid spot of Hills, the like of which I never yet saw, but shall endeavour therewith to give the Society a further Entertainment. Tours, &c. Account