ice, that ice defcends. In defcending, both the drop of water, and the grain of ice, are augmented by particles of the vapour they pafs through in falling, and which they condense by their coldness, and attach to themselves.

It is poffible that, in fummer, much of what is rain, when it arrives at the furface of the earth, might have been fnow, when it began its defcent; but being thawed, in paffing through the warm air near the furface, it is changed from fnow into rain.

How immenfely cold must be the original particle of hail, which forms the center of the future hailstone, fince it is capable of communicating fufficient cold, if I may fo fpeak, to freeze all the mafs of vapour condenfed round it, and form a lump of perhaps fix or eight ounces in weight!

When, in fummer time, the fun is high, and continues long every day above the horizon, his rays ftrike the earth more directly, and with longer continuance, than in the winter; hence, the furface is more heated, and to a greater depth, by the effect of those rays.

When rain falls on the heated earth, and foaks down into it, it carries down with it a great part of the heat, which by that means defcends ftill deeper.

The mafs of earth, to the depth perhaps of thirty feet, being thus heated to a certain degree, continues

continues to retain its heat for fome time. Thus the first fnows that fall in the beginning of winter, feldom lie long on the furface, but are foon melted, and foon absorbed. After which, the winds that blow over the country on which the fnows had fallen, are not rendered fo cold as they would have been by thofe fnows, if they had remained. And thus the approach of the feverity of winter is retarded; and the extreme degree of its cold is not always at the time we might expect it, viz. when the fun is at its greateft diftance, and the day fhorteft, but fome time after that period, according to the English proverb, which fays, "as the day lengthens, the cold ftrengthens;" the caufes of refrigeration continuing to operate, while the fun returns too flowly, and his force continues too weak to counteract them.

During feveral of the fummer months of the year 1783, when the effect of the fun's rays to heat the earth in thefe northern regions fhould have been greateft, there exifted a conftant fog over all Europe, and great part of North America. This fog was of a permanent nature; it was dry, and the rays of the fun feemed to have little effect towards diffipating it, as they easily do a moist fog, arifing from water. They were indeed rendered fo faint in paffing through it, that when collected in the focus of a burning glafs, they would fcarce kindle brown paper.

Of course, their fummer effect in heating the earth was exceedingly diminished.

Hence the furface was early frozen.

Hence the firft fnows remained on it unmelted, and received continual additions.

Hence the air was more chilled, and the winds more feverely cold.

Hence perhaps the winter of 1783-4, was more fevere, than any that had happed for many

years.

The cause of this univerfal fog is not yet afcertained. Whether it was adventitious to this earth, and merely a fmoke, proceeding from the confumption by fire of fome of those great burning balls or globes which we happen to meet with in our rapid courfe round the fun, and which are fometimes feen to kindle and be de

ftroyed in paffing our atmosphere, and whose fmoke might be attracted and retained by our earth or whether it was the vast quantity of fmoke, long continuing to iffue during the fummer from Hecla in Iceland, and that other volcano which arofe out of the fea near that ifland, which smoke might be spread by various winds, over the northern part of the world, is yet uncertain.

It seems however worth the enquiry, whether other hard winters, recorded in history, were preceded by fimilar permanent and widely extended fummer fogs. Because, if found to be

fo, men might from fuch fogs conjecture the probability of a fucceeding hard winter, and of the damage to be expected by the breaking up of frozen rivers in the fpring; and take fuch measures as are poffible and practicable, to fecure themselves and effects from the mifchiefs that attended the last.

PASSY, May 1784.

A fhort ACCOUNT of an EXCURSION through the SUBTERRANEOUS CAVERN at PARIS. By Mr. THOMAS WHITE, Member of the Royal Medical Society of Edinburgh, &c. &c. in a LETTER to his FATHER. Read February 9, 1785.

I

PARIS, July 29, 1784.

YESTERDAY vifited a moft extraordinary fubterraneous Cavern, commonly called the Quarries. But before I give you the history of my expedition it will perhaps be neceffary to fay a few words concerning the obfervatoire royal, the place of descent into this very remarkable cavern. This edifice is fituated in the Fauxbourg St. Jacques, in the highest part of the city. It takes its name from its ufe, and was built by Louis XIV. in 1667, after the defign of

Claude

Claude Perrault, Member of the Academy of Sciences, and first architect to his majesty. It ferves for the refidence of mathematicians, appointed by the king, to make obfervations, and improve astronomy. The mode of building it is ingenious, and admirably contrived, it being fo well arched that neither wood nor iron are employed in its conftruction. All the ftones have been well chofen, and placed with an uniformity and equality which contribute much to the beauty and folidity of the whole edifice. It is reckoned to be about eighty or ninety feet in height, and at the top there is a beautiful platform, paved with flint ftones, which commands an excellent view of Paris, and its environs. In the different floors of this building, there are a number of trap-doors, placed perpendicularly over each other, and, when thefe are opened, the ftars may be very clearly diftinguifhed, from the bottom of the cave, at noon day.

At this place, I was introduced to one of the infpectors, (perfons appointed by the king to fuperintend the workmen) by my friend Mr. Smeathman, who had ufed great application and intereft for permiffion to infpect the quarry, and had been fortunate enough to obtain it. For as this cavern is extended under a great part of the city of Paris, and leaves it in fome places almost entirely without fupport, the inspectors are very particular as to fhewing it, and en

deavour