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in the Middle of the Tube, the Plan or Ruler wherein it is fixed is level. When it is not level, the Bubble will rife to one End. This Glafs Tube may be fet in ano- ther of Brafs, having an Aperture in the Middle, whence the Bubble of Air may be obferved. The Liquor where- with the Tube is fill'd, is ordinarily either Oil of Tartar, or Aquafecunda; thofe not being liable to freezing as com- mon Water, nor to Rarefa&ion and Condenfation as Spi- rit of Wine. The Invention of this Inflrument is afcribed to Mr. Thevenot.
Air Lei-el with Sights, is an Improvement on that laft defcribed, which by the Addition of more Apparatus, be- comes more commodious and exa£t : It confitts of an Air Level i, (Plat. Survsying,Fig. 4.J about eight Inches long, and feven or eight Inches in Diameter, fet in a Brafs Tube 2, with an Aperture in the Middle. The Tubes are carried in a ttrong ttreight Ruler, a Foot long, at whofe Ends are fixed two Sights exactly perpendicular to the Tubes, and of an equal Height, having a fquare Hole, form'd by two Fillets of Brafs, crofting each other at right Angles, in the Middle whereof is drilled a very little Hole ; through which little Hole, a Point on a Level with the Inttru- ment is defcried. The Brafs Tube is fattened on the Ruler by means of two Screws, one whereof, mark'd 4, ferves to raife or deprefs the Tube at pleafure, for bring- ing it Towards a Level. The Top of the Ball and Socket is riveted to a little Rule that fprings, one End whereof is faften'd with Screws to the great Ruler, and the other End has a Screw 5, ferving to raife and deprefs the In- ftrument when nearly level. This Instrument is yet lefs commodious than the following one, becaufe tho* the Holes be ever fo fmall, yet they will ftill take in too great a Space to determine the Point of Level precifely.
Air Level with Telefcope Sights. This Level (Plate Survey- ing, Fig.5.') is like the lait Plate Fig. with this Difference, that inttead of plain Sights, it carries a Telefcope to determine exaftly a Point of Level at a good diftance. The Tele- fcope is in a little Brafs Tube, about 1 5 Inches long, fattened on the fame Rule as the Level. At the End of the Tube of the Telefcope, mark'd 1, enters the little Tube r, carrying the Eye-Glafs, and a Hair horizontally placed in the Focus of the Object-Glafs 2 ; which little Tube may be drawn out, or pufh'd into the great one, for adjufling the Telefcope to different Sights. At the other End of the Telefcope is placed the ObjecVGlafs : The Screw 5, is for raifing or lowering the little Fork carry- ing the Hair, and making it agree with the Bubble of Air when the Inflrument is level -, and the Screw 4, is for making the Bubble of Air agree with the Telefcope. The whole is fitted to a Ball and Socket. M. Huygens is faid to have been the Inventor of this Level-, which has this Advantage, that it may be inverted, by turning the Ru- ler and Telefcope half round : And if then the Hair cut the fame Point that it did before the Turn, 'tis a Proof the Operation is juft. It may be here obferved, that one may add a Telefcope to any kind of Level, by applying it upon, or parallel to the Bafe or Ruler, when there is occafion to take the Level of remote Objects.
Plumb orPendulum Level: That which fhews the Hori- zontal Line by means of another Line perpendicular to that defcribed by its Plummet or Pendulum. This Inftrument (Plate Surveying, Fig. 6.) confitts of two Legs or Branches, join'd together at Right Angles ; whereof that which car- ries the Thread and Plummet, is about a Foot and an half long. This Thread is hung towards the Top of the Branch, at the Point 2. The Middle of the Branch where the Thread paffes is hollow, that fo it may hang free every where but towards the Bottom at the Place 3, where there is a little Blade of Silver, whereon is drawn a Line perpendicular to the Telefcope. The faid Cavity is cover'd by two Pieces of Brafs, making, as it were, a kind of Cafe, left the Wind fhould agitate the Thread ; for which reafon the Silver Blade is covered with a Glafs, to the end that it may be feen when theThread and Plum- met play upon the Perpendicular. The Telefcope 1, is fattened to the other Branch or Leg of the Inftrument, and is about two Foot long, having a Hair placed horizontally a-crofs the Focus of the Objecl-Glafs j which determines the Point of Level, when the String and Plummet hang againft the Line on the Silver Blade. All the Accuracy or* this Inftrument depends on the Telefcope's being fitted at Right Angles to the Perpendicular. It has a Ball and Socket, by which it is faften'd to its Foot, and is faid to have been the Invention of M. Picard.
RefleBing Level: That made by means of a pretty long Surface of Water, reprefenting the fame ObjccT: inverted, which we fee erect by the Eye ; fo that the Point where thofe two Objects appear to meet, is in a Level with the Place where the Surface of Water is found. This is the Invention of M. Marriotte. There is alfo another RefeHhig Level, confifting of a Mirror of Steel or the like, well polifh'd, and placed a little before the Objecl-Glafs of a
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Telefcope fufpended perpendicularly. This Mirror mad make an Angle of 45 Degrees with the Telefcope ; in which Cafe the Perpendicular Line of the faid Telefcope is converted into an Horizontal j which is the fame with the Line of Level. The Invention of this is owing to Mr. Caffini. 5
We have another Level of Mr. Huygens's Invention, con- fifting of a Telefcope a, ( Plate Stirveying,Fig.-j .) in form of a Cylinder, going through a Ferril, in which it is faften'd by the middle. This Ferril has two flat Branches b h, one above, and the other below ; at the erids whereof are faften'd little moving Pieces, which carry two Rings, by one of which the Telefcope is fufpended' to a Hook at the end of the Screw 3, and by the other a pretty hea- vy Weight is fufpended, in order to keep the Telefcope in Equilibria. This Weight hangs in the Box 5, which is almoft fill'd with Linfeed Oil, Oil of Wallnuts, or other Matter that will not eafily coagulate, for more aptly fet- tling the Ballance of the Weight and Telefcope. The Inftrument carries two Telefcopes clofe and very parallel to each other, the Eye-Glafs of the one being againft the Obje£t-Glafs of the other, that one may fee each way without turning the Level. In the Focus of the Objeft-Glafs of each Telefcope mutt a little Hair be ttrain'd horizontally, to be rais'd and lower'd as occafion requires, by a little Screw. If the Tube of the Tele- fcope be not found level when fufpended, a Ferril or Ring 4 is put on it, to be Hid along, till it fixes to a Le- vel. The Hook on which the Inftrument is hung, is fiVd to a flat Wooden Crofs, at the Ends of each Arm where- of, there is a Hook ferving to keep the Telefcope from too much Agitation in ufing or in carriage. To the faid flat Crofs is applied another hollow Crofs, that ferves as a Cafe for the Inftrument ; but the two Ends are left open, that the Telefcope may be fecured from the Wea- ther, and always in a condition to ufe. The Foot of this Inftrument is a round Brafs Plate, to which are faften'd three Brafs Ferrils moveable by means of Joints wherein are put Staves : And on this foot is placed the Box. See the Figure*
Mafons Level, is cornpofed of three Rules, fo joined as to form an Ifofceles Reftangle,fomewhat like a Roman A ; at the Vertex whereof is fattened a Thread, from which hangs a Plummet 5 which paffes over a fiducial Line marked in the Middle of the Bafe, when the thing to which the Level is applied, is horizontal 5 but declines from the Mark, when the Thing is lower on one Side than the other.
Carpenters and Pavioiirs Level, confifts of a long Ruler, in the Middle whereof is fitted, at Right Angles, another fomewhat bigger, at the Top whereof is fattened a Line with a Plummet; which, when it hangs over a fiducial Line at Right Angles with the Bafe, iliews that the faid Bafe is horizontal. Thefe two laft Levels, tho* very common, are efteemed the beft for the Practice of Build- ing, tho' the Operations made by 'em mutt needs be fhorr.
Gunners Level, for levelling Cannons and Mortars, is an Inftrument (P late Surveying, Fig. 8.) confiding of a Triangu- lar Brafs Plate about four inches high, at the bottom of which is a Portion of a Circle divided into 450 ; which Number is fufficient for the higheft Elevation of Cannons and Mortars, and for giving Shot the greateft Range. On the Center of this Segment of a Circle is fcrewed a piece of Brafs, by means whereof it may be fiVd or moved at pleafure. The End of this Piece of Brafs is made fo as to ferve for a Plummet and Index, in order to /hew the Degrees of different Elevations of Pieces of Ar- tillery. This Inftrument has alfo a Brafs Foot to fet upon Cannon or Mortars, fo as when thofe Pieces are ho- rizontal, the whole Inflrument will be perpendicular. The Ufe of the Inflrument is obvious ; and confitts in placing the Foot thereof on the Piece to be elevated, in fuch man- ner as that the Point of the Plummet may fall on the pro- per Degree : this is what they call levelling the Piece.
Artillery Foot Level, Is in form of a Square, having its two Legs or Branches of equal Length ; at the Juncture whereof is a little Hole, whence hangs a Thread and Plummet playing on a perpendicular Line in the Middle of the Quadrant : it is frequently divided into 90 Degrees, or rather into twice 45 Degrees from the Middle. This Inflrument may be ufed on ordinary Occafions, by placing the Ends of its two Branches on a Plane ; for when the Thread plays perpendicularly over the middle Divifion of the Quadrant, that Plane is affuredly level. To ufe it in Gunnery, place the two Ends on the Piece of Artillery, which you may raife to any propofed Height by means of the Plummet, whofe Thread will give the Degree above the Level.
LEVELLING, an Operation with a Level, for finding a Line parallel to the Horizon, at one or more Stations, in order to determine the Height of one Place with refpeci to another ■> for the laying off Grounds even, regulating
of
