gang of robbers, who infefted the country for many miles round the city of Paris.

As to the origin of this quarry, I could not, on the ftricteft inquiry, learn any thing fatisfactory; and the only account I know published, is contained in the Tableaux de Paris, Nouvelle edition, tom premier, chapitre 5me. page 12me.

"Pour batir Paris dans fon origine, il a fallu "prendre la Pierre dans les Environs; la con"fommation n'en a pas êté mince. Paris s'agrandiffant on a a bâti infenfiblement les "Fauxbourgs fur les anciennes Carriers, de "forte que tout ce qu'on voit en dehors, man

que effentiellement dans la terre aux Fonde"ment de la Ville; de la, les Concavites effray

antes, qui fe trouvent aujound'hui fous les "maisons de plufieurs quartiers; elles portent "fur les Abymes. Il ne foudroit pas un choc "bien confiderable, pour ramener les pierres au

point d'ou on les a enleves avec tout d'effort. "Huit perfonnes enfevelies dans un Gouffre de "cent cinquante Pieds de Profondeur, et quel

ques autres accidens moins connus, ont excité "enfin la vigilance de la Police, et du gou

vernement; & de fait, on a etagé en filence "les edifices de plufieurs quartiers, en leur "donnant dans ces obfcurs Souterreins un apui "qu'ils n'avoient pas.

"Tout le Fauxbourgs Saint Jacques, la Rue "de la Harpe, & meme la Rue de Tournon,

portent

56 portent fur d'anciennes Carriers, & l'on a "bati des Pilaftres pour foutenir le Poids des << Maifons. Que de matiere a reflexions, en "confiderant cette grande ville formée, & fou"tenue par moyens abfolument contraires! Ces "Clochers, ces Vontes des temples, autant "de fignes, qui difent a l'oeil ce que nous "voyons en l'air manque fous nous Pieds."

"For the first building of Paris, it was neceffary to get the ftone in the environs, and the "confumption of it was very confiderable. As "Paris was enlarged, the suburbs were infenfibly "built on the ancient quarries, fo that, all that "you fee without is effentially wanting in the "earth, for the foundation of the city hence proceed the frightful cavities, which are at "this time found under the houfes in feveral quarters. They stand upon abyffes. It would "not require a very violent fhock to throw back "the ftones to the place, from whence they "have been raised with so much difficulty.

re

Eight men being fwallowed up in a gulph one "hundred and fifty feet deep, and fome other "lefs known accidents, excited at length the "vigilance of the Police and the government, " and, in fact, the buildings of feveral quarters "have been privately propped up; and by this "means, a fupport given to thefe obfcure fub"terraneous places, which they before wanted. VOL. II. B b

" All

"All the fuburbs of St James's, Harp-street, "and even the street of Tournon, ftand upon "the ancient quarries; and pillars have been "erected to fupport the weight of the houses. "What a subject for reflections, in confidering "this great city formed, and fupported by " means abfolutely contrary! These towers, "these steeples, the arched roofs of these temples "are fo many figns to tell the eye, that what "we now see in the air, is wanting under our "feet."

A DESCRIPTION of a NEW INSTRUMENT for meafuring the SPECIFIC GRAVITY of BODIES. By Mr. WILLIAM NICHOLSON, in a LETTER to Mr. J. H. MAGELLAN. F. R. S. Reg. Acad. Petropol.& Paris. Correfp. &c. Read May 4, 1785.

DEAR SIR,

A

CCORDING to my promife, I tranfmit to you an account of the Inftrument I have conftructed for the easy and exact finding the fpecific gravities of bodies. It appears to me to be as perfect, as the nature of a floating inftrument of this kind will admit of; and, for that reason, I prefume it will not be impertinent to

mention

mention previously what has been done in this way.

It seems to follow from a paffage in Boyle's ⚫ account of a new Effay Inftrument, that the Hydrometer, or Areometer was first invented by that great philofopher. The effay inftrument here mentioned, was intended for the hydroftatical proof of metals, and was adapted to ferve chiefly for guineas. It confifted of a ball, fomewhat less than an hen's egg, with a ftem of four or five inches in length, foldered to the upper part, and a bent wire or stirrup beneath, to place the coin upon. A flit piece of brafs, with a lateral fcrew to hold the coin tight, though in fact conducing more to the ease than accuracy of the experiment, is mentioned by the author, as being preferable to the ftirrup: and, to extend the use of the inftrument, he propofes that the ball be made large, and provided with a contrivance for occafionally changing the quantity of ballaft applied beneath the ball.

Boyle's inftrument was intended to be used in water, and, confequently, the graduations of its ftem denoted certain invariable weights. But when the hydrometer is to be used in various fluids, it diminishes the accuracy of the results,

Lowthorp's abridgment of the Philofophical Tranfactions, vol. I. p. 604. Or Boyle's Works in 4to. edit. London, 1772, vol. IV. p. 204.

if those spaces be taken for abfolute weights; or, at all events, it brings forward a rather intricate confideration of the relation which the bulks of the spaces, or parts of the ftem, have to the whole immerfed part. This appears to have been the inducement that led M. G. Fahrenheit* to add a fmall difh or fcale to the top of the upper ftem, which, instead of graduations, had only a fingle mark that, in all cafes, was to be brought to the surface of the fluid, by means of weights added in the faid fcale. Mr. Clarke, who in the year 1730 published an account of an hydrometer, does not appear to have been apprised of what had been done before by Boyle and Fahrenheit. For he fpeaks of his own inftrument as a new invention, though it does not differ from that of Boyle, except in having a great number of ballaft weights to be screwed occasionally to the lower stem, instead of depending on the graduations of the upper ftem; and he affirms, that the specific gravities. of fluids cannot be found without a great deal of trouble, though it is certain that they may be found with greater eafe, and much more accuracy, by that of Fahrenheit, than by his own. Clarke's hydrometer, with weights adapted to

Reid and Gray's Abridgment of the Phil. Tranf. vol. VI. part I. p. 294.

+ Ibid. vol. VI. part I. p. 295.

allow