bright but not very vivid yellow to abrown yellow, called spruce ochre, and is always of a warm cast. Its natural variety is much increased by artificial dressing and compounding.

Yellow Orpiment, or Yellow Arsenic, is a sulphurate oxide of arsenic, of a beautiful, bright, and pure yellow colour, not extremely durable in water, and less so in oil. In tint with white lead, it is soon destroyed. It is not subject to discoloration in impure air.

Yew. The yew-tree is common in Spain, Italy, and England, and is indigenous to Nottinghamshire. The tree is not large, and the wood is of a pale yellowish-red colour, handsomely striped, and often dotted like amboyna. It has been long famed for the construction of bows, and is still so employed. The English species is a hard, tough, and durable wood, and lives to a great age. It is also used for the making of chairs, the handles of articles of furniture, &c.

ZEB

ZEBRA WOOD is the produce of the Brazils and Rio Janeiro; it is sent in logs and planks as large as 24 inches. The colour is orange brown and dark brown variously mixed. Its beautiful appearance fits it for cabinet-work and turnery. Zero, the commencement of a scale marked 0, or nothing. It usually denotes the point from which the scale of a thermometer is graduated. Zeta, presumed to be a room over the porch of a Christian church Zigzag, a moulding by lines arranged

in the manner of the heraldic chevron. Zigzag is found frequently used in Norman and Anglo-Norman architecture. Very many beautiful specimens of this ornament exist in doors and windows of the Anglo-Norman Gothic in England. (See Doors, and the Frontispiece for an early specimen.) Zinc. This metal exists in abundance,

and is employed for many purposes. It is commonly combined with sulphur in zinc blende, and with carbonic acid in the mineral calamine, which is the most valuable of all its ores. Zinc may be obtained pure by re-distilling in a porcelain retort, which is sold in commerce. It is obtained from the ores as follows: they are first washed and mixed with powdered coke or charcoal, are distilled from

ZIN

an earthen close vessel, with an iron tube passing through its bottom, the upper end of which is open, and the lower end entering a vessel of water.

At a bright red heat zinc volatilizes, and is condensed in the water, gases passing off along with it. It is a bluish-white metal, which slowly tarnishes in the air. It is brittle at ordinary temperatures, but at about 300° it is malleable, and may be rolled or hammered into sheets, and retains its ductility when cold. At 400° it

may be reduced to powder: it melts at 773°.

Zinc white is an oxide of zinc which

has been more celebrated as a pigment than used, being perfectly durable in water and oil, but wanting the body and brightness of fine white leads in oil; while, in water, constant or barytic white and pearl white are superior to it in colour, and equal in durability. Nevertheless, zinc white is valuable as far as its powers extend in painting, on account of its durability both in oil and water, and its innocence with regard to health; and, when duly and skilfully prepared, the colour and body of this pigment are sufficient to qualify it for a general use upon the palette, although the pure white of lead must merit a preference in oil.

Zocle, a name given to a low, plain, square member or plinth supporting a column

Zoophorus, in architecture, a part between the architraves and cornice, so called on account of the

ornaments carved on it, among which are the figures of animals Zyghyr, or Sigger, in mining. When a slow stream of water issues through a cranny, it is said to sigger, or zyghyr.

ANGLE-STAFFS or Staff-heads, vertical heads, generally of wood, fixed to exterior angles of a building, flush with the surface of the plaster, on both sides, for the purpose of fortifying them against accident. They serve also for floating the plaster. Their section is about three-fourths of a circle, with a projecting part from the other quarters, by which they are fastened to the wood-bricks, plugging or bond timber.

Angular modillions, those which are placed at the return of a cornice in the diagonal vertical plane, passing through the angle or mitre of the cornice Electric Telegraph. The employment of electricity in the transmission of intelligence originated at an early period of the history of electrical science. Plans to this effect had been brought before the public; but all wanted a simplicity of principle and of construction. In 1837, Messrs. Cooke and Wheatstone obtained their first patent for an electric telegraph, applicable to general purposes. This patent has been subsequently followed at short intervals by others, in which the invention has been gradually brought to its present form; the principles originally employed have been progressively rendered more varied and general in their application, and the apparatus more simple in its details. By these improvements the number of wires necessary for the conveyance of intelligence has been reduced, and the construction has been rendered cheaper and more perfect.

The electric telegraph involves

in its construction two essential principles. First, that a magnetized needle, which is free to rotate about its centre, being brought near to a wire, through which an electric current is passing, has a tendency to place itself at right angles to that wire; the direction of its motion following a certain invariable law. This fact was the discovery of Prof. Ersted, of Copenhagen, in 1819. Secondly, that a piece of soft iron, not being permanently magnetic, is rendered temporarily so, during the transmission of an electric current along a wire coiled spirally around it.

The figures to which reference is here made, in the brief description of the apparatus, are, 1. A view of the interior of the singleneedle instrument, showing the position of the coil and of the battery connections. 2. A vertical section of the same, through the coil and handle. 3. The handle or key, in the position for giving a signal, part being removed, to render the battery connections more distinct. 4. Plan of the same. The double-needle instrument differs from the single-needle only in the duplication of all the parts.

The coil A, figs. 1 and 2, consists of a light hollow frame of brass or wood, around which are wound, in two portions, about 200 yards of fine copper wire, covered with silk or cotton. This length of wire renders the indications of the needle distinct and prompt, even with a low-battery power, or when forming part of a very extended circuit. The resistance which would be offered by the fine wire of the coil

(its diameter being about th of an inch) to the passage of a current of electricity, derived from an ordinary battery of a few cells only, is overcome by using a battery arrangement of considerable intensity, but which develops the electrical fluid only in small quantity. Or, speaking rather more correctly, we should say, that the electromotive resistance, both of the battery and also of the ordinary circuit, being very considerable, the introduction of the resistance of the coil into the latter produces but little influence upon the transmission of the current. Within the brass frame, and therefore interior to the coils of wire, is suspended a magnetic needle, upon a horizontal axis b, which passes across the middle of the frame, and turns on fine pivots at the back and front of the coil. In front of the frame and of the dial of the instrument is fixed on the same axis b, a second needle having its poles oppositely placed to those of the first. This outer needle serves as the indicator or pointer, by which the signals are made, and at the same time is acted upon by the coil, though in a less degree than the inner needle. The combination of the two needles being thus rendered astatic, it is necessary to give a slight preponderance to their lower ends, in order that they may recover their vertical position, after having been deflected; and the action of gravity has been found more effectual, in bringing the needle to rest without oscillations, than either springs applied at the sides, or the directive influence of permanent magnets. With the coil and needles thus arranged, it is evident that signals may be given by the combination of successive deflections to one side or the other; the extent of such deflections being limited to any degree that may be found convenient, either by pins fixed on the dial of the instrument, or by stops

placed at the sides of the brass frame of the coil. In fact, all that is necessary for rendering these movements of the needle available for the transmission of intelligence, is a contrivance for reversing with ease and rapidity the connection of the battery with the ends of the two conducting wires. This expedient is provided by the handle or key of the instrument.

The conductor through which the electrical current is to circulate must be absolutely complete in all parts. It is not necessary that the material of the circuit be the same throughout, but only that its conductibility be maintained from the one pole of the battery to the other; the slightest want of continuity of the conducting matter, at any part of the circuit, being fatal to the passage of the fluid. So long as the wires for telegraphic purposes were extended between the two points of communication, by being laid within tubes buried in the earth, a second wire was requisite to enable the current to return from the distant station to the point whence it set out. It was well known that the earth itself afforded such a means of return; but the insulation of the wires in the tubes from the earth, could not be rendered sufficiently perfect, to make the use of the earth, as a portion of the circuit, either prudent or desirable. When, however, the wires were suspended in the air, according to Mr. Cooke's patent of 1842, the earth was advantageously employed as half the circuit. All that was found necessary, was to connect the extreme ends of the conducting wire with plates of copper or other metal of two or three feet of surface, buried at some depth in the ground; or with any system of gas or waterpipes, which might afford a continuous metallic path for the fluid to the earth. In either case, the depth of the connection beneath