Next, fet 100 on B to the reserved number on A, and against the whole content on B will be found the ullage on A.

There are many folid figures befides the foregoing, the contents of which cannot be found by any actual dimenfions taken on the figure, such as statues, &c. The contents of fuch bodies are found by immerfing them in water, and meafuring the rife of the water occafioned by fuch immersion.

SECT. V.

A VIEW OF THE FIVE ORDERS OF ARCHItecture.

The origin of architecture is almost contemporary with that of civil fociety. When mankind felt the inclemencies of the weather, and, confequently, found the neceffity of erecting habitations for shelter, eafe and convenience foon improved itself into ornament and grandeur. A few trees perhaps growing round in a circle, and leaning together at the top, afforded the firft habitations, being interwoven with twigs, and plastered over with mud. This gave rife, in after-ages, to the idea of coluinns.

It is probable that the inconvenience of these habitations rendered their owners defirous of inventing such as should answer every purpose of the former fort, and poffefs feveral advantages above those: this improvement naturally gave rife to the invention of the cross beams to fupport the roof; for this purpose, they had, no doubt, recourse to the trunks of trees: thus columns were at once introduced.

VOL. I.

3 F

The

The Egyptians were the first people from whom the Greeks derived all their knowledge of architecture; as ap pears from those ancient remains, the pyramids; and from the ruins of a magnificent temple at Luxor, the ancient Thebes, of which there remain more than 130 columns, all of confiderable magnitude; fome 10 feet in diameter, with elegant sculpture: and feveral other remains in the neighbourhood of the fame place.

The fertile genius of the Greeks foon improved upon the ponderous architecture of Egypt, whofe ignorance in conftructing the arch made it neceffary for the columns to stand very thick, with large and flrong architraves.

The Tufcans (or Etrufcans) were also very skilful in building; from whom the Romans received great help in architecture, in all their large works: the temple of Jupiter Capitolinus, the Cloaca Maxima, &c. are the works of Tuscan artists.

To produce the authority for every fpecies of ornament in each of the five orders, would be impoffible, at the prefent day; I fhall, therefore, point out the more general parts, with the origin of their introduction.

1. The PLINTH was, no doubt, at firft a fquare stone, or tile, placed under the tree or primitive column, only to prevent its rotting by the conftant moisture of the earth, and also to serve as a folid footing to the column.

2. The TORUS is the fwell above the plinth, and originated, no doubt, from the form of the tree, being thicker at the bottom than the part above, by which means it was fixed more firmly on the plinth: by fome it was conjectured to have been a rope or bandage only, round the trunk, to prevent its splitting.

3. The CAPITAL had its origin from the parts of the tree where the arms were cut off: the fwell of the arms gavé the idea of the fwell of the capital," which is attended with this advantage, by being broader at the top, it is better formed for fuftaining the works above.

4. The ABACUS was ne more than a tile or stone to throw off the water, and to prevent its finking into the column.

5. The ASTRAGALS and FILLETS were bandages to bind the column.

16. The ARCHITRAVE was beams neceffary to hold or unite the columns together.

7. The FRIZE was only an addition, when height was wanted in the inner parts.

8. The CORNICE, with its ornaments, were only the ends or edges of the timbers, neceffary to fupport the roof.

9. The DENTELES, TRIGLYPHS, MODILLIONS, &C. were accidental improvements added in after-ages, when structures of wood gave place to those of stone.

The five orders of architecture are, the Tufcan, Doric, Ionic, Corinthian, and the Composite.

The TUSCAN order is diftinguished by its plain and robust appearance it is used only where strength and plainness are required. The Trajan column at Rome is an instance of its effect and elegance.

The DORIC has nearly the fame character for strength as the Tufcan, but it is enlivened by several ornaments. It is ufed with good effect where united strength and grandeur are required.

The IONIC has more delicacy in the construction than either of the former orders; it partakes of a middle order, between the masculine Tuscan and Doric, and the flender Corinthian the capital is diftinguished by its boldness, and the shaft by its beauty, which makes it applicable to porti coes, frontispieces, entrances to houses, &c.

The CORINTHIAN has more ornament and delicacy than any other order: the capital is diftinguished by its beauty and richness, and the pillar by its delicacy: which renders it proper, where elegance only is required. It is frequently used for internal decorations, in ftate rooms, &c.

The COMPOSITE is only the Corinthian, with the modern Ionic volute to the capital, which gives it a bolder projection; the proportions and effects are the fame as those of the Corinthian, and it is applicable to the fame uses.

The measurements in architecture are taken in minutes; one half of the lower diameter of the column is usually di vided into thirty parts or minutes. The projections are meafured from the perpendiculars of the superior and inferior parts of the column.

CHAP. VIII.

OF OPTICS.

I.

Definitions.

OPTICS is that science which treats of the nature, properties, and laws of vifion: and in its most extensive sense, comprehends the whole doctrine of light and colours, and all the phenomena or appearances of vifible objects. Optics confifts of three parts: viz. catoptrics, dioptrics, and chro

matics.

2. CAT

2. CATOPTRICs is that part which treats of reflected vifion, or that which relates to viewing objects by reflected light, as in mirrors. This is called reflex vifion.

3. DIOPTRICS treats of viewing objects through a different medium or body; as through glass, water, &c.

4. CHROMATICs treats of the different colours of light. 5. LIGHT is that property by which bodies are rendered vifible. It confifts of an infinite number of extremely small particles, which flow from every body expofed to it, and in Straight lines.

6. A RAY or BEAM OF LIGHT is a very fmall part of it, proceeding from the luminous body.

7. The RADIANT is the object from which the rays of light proceed.

8. The SPECIES of an object is the image of it, made by the rays of light, in the focus of the reflecting or refracting medium..

9. PARALLEL RAYS are fuch as proceed from a body, parallel to each other.

10. CONVERGING RAYS are thofe rays, which, proceeding from a body, approach nearer each other in their progrefs, and tend to one certain point: thus the rays, proceeding from LN to the point O, are said to converge to that point. (Fig. 5. plate 5.)

II. DIVERGING RAYS are those which, proceeding from any point, do continually recede from each other in their progrefs: thus the rays proceeding from O towards LN are faid to diverge, (Fig. 5.)

12. The Focus of rays is that point to which all converging rays tend, as O (fig. 5): this is called the real focus.

13. The VIRTUAL or IMAGINARY FOCUS is a point, as f, to which the rays A B tend, and where they would unite, if they were not intercepted by the obstacle CD (which suppose a mirror), by which means they are turned afide, and made to converge in a real focus, E. (Fig. 6.)