IN compliance with your desire, I made particular inquiry, whether at or about the time the earthquake happened at Lisbon the 1st of November 1755. any uncommon phænomena were observed to appear in the islands of Orkney or Zetland, as such had happened about that time in other parts of Scotland. From Orkney I was informed, that nothing particular had happened; only, that about the time mentioned the tides were observed to be much higher than ordinary. I received from Zetland a letter, dated 28th May 1756. from Mr. William Brown, Master of the grammar-school at Scalloway in that country, a sensible and observing man; wherein he writes verbatim as follows. “Blessed be God, notwithstanding the great devastations, that have been made in other parts of the world by earthquakes, we have been intirely free from any disaster of that nature: nor has any thing extraordinary happened in this country since you left it; only on Monday the 20th October last, betwixt the hours of three and four in the afternoon, the sky being very hazy, as it uses to be before a storm of thunder and lightning, there fell a black dust over all the country, tho' in greater quantities in some places than in others. It was very much like lampblack; but smelled strongly of sulphur. People in the fields had their faces, hands, and linen, blackened by it. It was followed by rain.——Some people assign the cause of it to some extraordinary eruption of Hecla. But I shall trouble you no more about it, as no doubt some of your friends have written to you of it some time ago.”——
In June 1756. I returned to Zetland; and, upon further inquiry, found what Mr. Brown had written me was attested by Mr. Mitchell, parson of the parish of Tengwall, and by several Gentlemen of credit and reputation, who had seen and observed the same phænomenon in different parts of the country at the time above-mentioned.
Mr. Brown having omitted to mention, how the wind did blow at the time the black dust was observed, I made particular inquiry about that circumstance, and found it was from the S.W. which does not seem to favour the opinion, that the dust proceeded from an eruption of mount Hecla, which lies about N. W. from Zetland; unless it may be supposed, that a north wind happening just before had carried this dust to the southward, and the south-west wind immediately following had brought it back to the northward. But, in this case, would not this black dust have been observed in Zetland at its first travelling to the southward? Upon inquiry, I did not hear it was.
Thus far I have obeyed your commands, which I will always do with pleasure; and if you think it worth while to lay this letter before the Royal Society, I leave you at full liberty to do so, or not, as you think proper: but what it contains may be relied on as truth. I am, with great regard,
Dear Sir,
Your most obedient humble Servant,
And. Mitchell.
P.S. I may add, that the distance from mount Hecla to Zetland is between 500 and 600 miles.
XXXVIII. A Description of some Thermometers for particular Uses. By the Right Honourable the Lord Charles Cavendish, V.P.R.S.
Read June 30, 1757.
THE thermometer ([Tab. XI.] fig. 1.) is designed for shewing the greatest degree of heat, which happens in any place during the absence of the observer. It consists of a cylinder of glass joined to a tube, and differs from common thermometers only in having the top of the stem drawn out into a capillary tube, which enters into a glass ball C, joined on to the stem at the place where it begins to be contracted. The cylinder, and part of the tube, are filled with mercury; the top of which shews the common degrees of heat as usual. The upper part of the tube above the mercury is filled with spirit of wine, and some of the same liquor is left in the ball C, so as to fill it almost up to the top of the capillary tube.
Now when the thermometer rises, the spirit of wine will be driven out of the tube, and will fall into the ball C. When the thermometer sinks again, as the spirit cannot return back from the ball, the top of the tube will remain empty, and the length of the empty part will be proportional to the fall of the thermometer. Therefore, by means of a proper scale, the top of the spirit of wine will shew how many degrees it has been higher than when observed; which being added to the present height, will give the greatest degree of heat it has been at.