Εικόνες σελίδας
PDF
Ηλεκτρ. έκδοση

not permanent; yet when well prepared and judiciously used in sufficient body, and kept from strong light, it has been known to last many years; but it ought never to be employed in glazing, nor at all in performances that aim at high reputation and durability. Scarp, in heraldry, signifies the scarf

worn by military commanders Scena, the permanent architectural front which faced the audience part of a Roman theatre: it sometimes consisted of three several ranges of columns one above another Scenography, in perspective, the representation of a body on a perspective plane; a description thereof in all its dimensions, such as it appears to the eye

Schola, the margin or platform surrounding a bath. It was occu

pied by those who waited until the bath was cleared. The schola was also a portico corresponding to the exedra of the Greek palæstra, and Iwas intended for the accommodation of the learned, who were accustomed to assemble and converse there.

Schools of Painting. A school in the

fine arts denominates a class of artists who have learned their art from a certain master, either by receiving his instruction or by studying his works, and who of consequence discover more or less of his manner from the desire of imitation, or from the habit of adopting his principles. All the painters which Europe has produced since the renovation of the arts are classed under the following Schools: the School of Florence, the School of Rome, the School of Venice, the Lombard School, the French School, the German School, the Flemish School, the Dutch School, the Spanish, and the English School.

Schooner, in navigation, a small two

masted vessel whose mainsail and foresail are suspended from gaffs and stretched out below by booms

Schweinfurt Blue appears to be the same in substance as Scheele's green, prepared without heat, or treated with an alkali. It is a beautiful colour, liable to the same changes and is of the same habits as blue verditer, and the above ineligible pigment. Sciography, in architecture, the profile or section of a building, to show the inside thereof. In astronomy, the art of finding the hour of the day or night by the shadow of the sun, moon, or stars. Sconce, in manufactures, a pensile candlestick, generally with a mirror to reflect the light

Scotia, the hollow moulding in the base of an Ionic column, derived from the Greek, signifying shade, because, from being hollow, part of it is always in shadow. The scotia is likewise a groove or channel cut in the projecting angle of the Doric

corona.

Scovanlode, a lode having no gozzan on its back or near the surface Scraper, a piece of iron used to take out the pulverized matter which remains in a hole when bored previous to blasting

Screen, a moveable frame-work to keep off an excess of light, or heat, or cold; a separation; a partition. In ecclesiastical architecture, a screen denotes a partition of stone, wood, or metal; usually so placed in a church as to shut out an aisle from the choir, a private chapel from the transept, the nave from the choir, the high altar from the east end of the building, &c. Some very beautiful examples exist of screens, especially of those separating the choir from the nave. That of York is of a magnificent character. All Saints' Church, Maidstone,' a work published in 4to, is of an interesting description on this head. In modern architecture, a single open colonnade, admitting a view through it, is called a screen of columns: such was that formerly in front of Carlton House.

Grosvenor House has a Doric

screen in front of it

Screen bulk-head, in ship-building,

that which is under the roundhouse Screw, a spiral groove or thread

or

The

winding round a cylinder so as to cut all the lines drawn on its surface parallel to its axis at the same angle. The spiral may be either on the convex or concave surface of the cylinder, and it is called accordingly either the screw the nut. The screw can hardly be called a simple machine, because it is never used without a lever or winch to move it home, and then it becomes an engine of amazing power and utility in pressing together substances that have little cohesion, or in raising to short heights ponderous bodies. smith, the carpenter, the printer, and the packer, all use screws in their respective occupations. Bales of wool, cotton, hay, &c., may be compressed by means of a screw into packages, the specific gravity of which shall be much heavier than an equal volume of water. Such packages will then sink in the ocean like a cannon-shot. Moreover, many of our domestic operations are performed by means of presses or screws; as the making of sugar, oil, and wine. The screw possesses one great advantage over the inclined plane, from which its principal of action may be said to be derived. The great attrition or friction which takes place in the screw is useful by retaining it in any state to which it has once been brought, and continuing the effect after the power is removed. It is thus the cabinet-maker's cramp, the smith's vice, and all those instruments made by mathematical instrumentmakers in which screws act, can be employed with certainty. Screws are made with threads of various forms some have sharp, others square or round threads. Screw-jack, a strong screw for lifting

:

or supporting a heavy weight: it rests, by means of a large nut, upon a hollow base or pedestal, and is raised or lowered by turning the nut.

Screw. Screw propellers, for navigation, by means of steam power, have now become objects of importance to all nations, more particularly for those who navigate the broad waters: they are especially applicable for vessels of war, the machinery for propulsion being without the reach of shot. Screw propellers, however variously they may be modified, all derive their power of propelling by being placed on an axis which is parallel to the keel, and by having threads or blades extending from the axis, which form segments of a helix or spiral, so that, by causing the axis to revolve, the threads worm their way through the water, much in the same way as a carpenter's screw inserts itself into a piece of wood. There is, however, considerable difference between the action of a carpenter's screw and of the screw propeller: the latter, acting upon a fluid, cannot propel the vessel without causing the water to recede, while the carpenter's screw progresses thro' the wood without any such recession. The law which governs the distance which the water recedes is common to the paddle-wheel, and to all bodies moving in the water. The screw propelling is not of recent construction; we find that so early as 1727, Mr. Duquet invented an hydraulic screw machine, which he placed between two boats, connected by transverse bearing, to which the screw was affixed. Mr. Paucton, in 1768, published his 'Theory of the Screw of Archimedes;' other inventions followed, until a recent date, when Mr. George Rennie applied his comprehensive mind to the subject. Sir John Rennie and Mr. George Rennie undertook, when all other engineers declined the

[merged small][merged small][merged small][merged small][merged small][ocr errors][graphic][merged small][merged small][subsumed]
[ocr errors][merged small][merged small][graphic][subsumed][merged small][merged small][merged small]

make a screw at his own cost, which they

[graphic]
[blocks in formation]

screw was made of
copper, having four
blades; and it corre-
sponded in every re-
spect with a screw
of an uniform pitch,
also made of copper,
in the number of its
blades, its diameter,
length and pitch, with
this exception, that
although they each
commenced with the
same pitch, yet Mr.
Woodcroft's screw
gradually increased
in its pitch through-
out, and terminated
with an increase of
5 per cent. additional
pitch. The trial of
Smith's four-bladed
Archimedean or true
screw took place in
the Rattler,' on the
18th of March, 1844;
and on the 13th of
the following month,
a trial of Wood-
croft's increasing-
pitch screw, of four
blades, was made
with the same vessel.

After this trial, Mr. Lloyd, the chief engineer of the Admiralty, who had been present to superintend it, stated to Mr. Woodcroft, who had also been present, that the latter screw was superior to the uniform pitch screw in two important qualities: first, that it propelled the vessel at an equal speed with less power; and, secondly, that it also propelled the vessel at an equal speed with fewer revolutions of the screw, which latter quality he considered superior to the former; but that the difficulty arising from having to drive screws so fast, constituted the greatest obstacle to their in

troduction in the Royal Navy. Indeed, this is the admitted difficulty in the application of the screw as a marine propeller, and the practical difference between it and the paddle-wheel. The great size of latter enables the speed of the engines to accomplish the required velocity of the boat, whereas the small diameter of the screw renders it necessary that it should perform many more revolutions than the engine makes strokes. Hence the necessity of introducing some multiplying gearing between the engine and the propeller; and this multiplying gear, consisting of cast-iron cog-wheels and pinions, is neces

« ΠροηγούμενηΣυνέχεια »