Letter 2727

Fol. 192 Hond Sr Upminster 13 Feb: 1730/1 The late Frost having been almost as in- tense as any that hath been for many years, I send you my Account of it: wch if you think worth the cogni- zance of the R.S. be pleased to impart it to them. In the Philos: Transact: for Novr. & Decr. 1709 326. I have given an Account of some of the most remarkable Frosts, that I could fin any relation of, & particularly of that great and I had almost said universal one in 1708, wch the Soc. had very good Histories of from divers parts, & wch, in that Transactions, I have given an account of from the original Papers, wch the Soc- iety as pleased to do me the honr to entrust me wth. In that Trans: I have made it very pro- bable, yt the greatest Descent of the Spirits in the Thermometer, was on Decr 30 1708, when y Glass was within one tenth of an inch low as it is wth Artificial Freezing with now or Ice & Salt. And in the late Frost was almost, if no altogether, as low. The Freezing-Point of my Thermometer Is 10 inches (wch I call 100 degrees) above the Globe of Spirits; & the most intense Freezing (ac- cording to the methods I have mentioned in at Transaction) is just at, or very little within the Ball. And on Jan. 30 about Sun-rising the Thermometer was but an inch, or 10gr above the point of extreme Freezing; & on Febr. 3, at only half an inch, or 5gr. And considering that Fol. 193 the Thermometer I observed with in 1708, was much less accurate, & differenlt graduated from that wch I now have, I am apt to think, that the Frost on Feb. 3 last, was altogether as intense as yt on Dec 30 1708. For although a Frigori- fick Mixture sunk the Spirits but one tenth lower in the Old Thermometer, & about 5 or 6 tenths in that I now observe with, yet I take the Difference to be little, or none at all, by reason of the tenderness of the New above the Old Glass. And this degree of Cold, I take to be as ex- cessive, as in any of the years mentioned in the said Transactions; yea any of the Years, what the Thames at London was frozen over: I am sure than in the year 1716, when that River was frozen over for several miles, and Booths & streets were made on the Ice; an Ox roasted there- on, &c. For the lowest point of Freezing in 1716 was on Jan 7, when the Spirits fell to 35gr on- ly of the Glass I now make use. But the true cause of the Freezing of the Thames yt year, was not barely the Excess of the Cold, but ye long continuance of it. Which also was the principal cause of those remarkable Congela- tions of yt River in 1683, & 1708, when I saw coaches driven over the Ice, large Fires made on it, &c. I am wth great respect Hond Sr Yours Wm Derham I have just rigged up & tryed the Reflecting 8 foot Telescope I had of Mr Molyneuxes grinding, & find it admirable.