DIVING-BELL PUMP.

rounded. The pump which supplies air to the reservoir is so constructed that liability to leakage diminishes with the pressure, and the air is compelled to traverse two layers of water before entering the reservoir, rendering it much cooler than it would otherwise be in its greatly compressed state; it is farther cooled by expansion in passing from the reservoir into the air-chamber.

An important advantage possessed by this apparatus is that the expired air rises in bubbles to the surface. So long as the diver breathes regularly, the intervals between the appearance of the bubbles is sensibly equal. If they come more rapidly or more slowly than usual, it is a sign that something abnormal is going on. If they cease altogether, the diver must have ceased breathing, and should be hauled up immediately.

In the old forms of diving-dress the air filled the space between the body of the diver and his impervious clothing, the expired air escaping by a small valve in the helmet, through which any excess of air also escaped. Irregularity in the action of the pump caused also irregularities in the escape of the bubbles, and thus the assistants might for a long time unconsciously continue to send air to a corpse. In the new apparatus, the appearance of the bubbles indicates the safety of the diver, and the assistants on the watch are at any time warned of his danger by their nonappearance.

The armor employed in connection with the breathing-apparatus only serves to defend the diver from cold, and may therefore be made much lighter, lowing greater freedom of motion. See ARMOR,

SUBMARINE.

into an exterior chamber, whence it is carried to a condenser, and thence, through a tube, to the bell. Diving - dress. A waterproof clothing and helmet for those who make submarine explorations. See ARMOR, SUBMARINE.

Di-vision-plate. The disk or wheel in the gearcutting lathe, which is pierced with various circular systems of holes; each circle represents the divisions of a circumference into a given number of parts.

Do'be-rein'er's Lamp. An instrument invented by Professor Dobereiner, in Jena, in 1824, for obtaining light by the projection of a jet of hydrogen upon a piece of spongy platinum. See HYDROGEN LAMP.

Dock. 1. (Hydraulic Engineering.) An artificial excavation or structure for containing a vessel for repairs, loading, or unloading. Docks are of various kinds. See

Div'ing-bell Pump. A pump having a casing divided by a vertical partition into two chambers, Fig. 1678. д

Diving-Bell Pump.

which are provided with inwardly and outwardly opening valves. The chambers are kept partially filled with water, which, together with air, is admitted to each through the inwardly opening valves, and expelled through those opening outwardly, to supply the bell with fresh air. This is effected by the alternate reciprocations of a piston working in the open-ended cylinder, which, at each stroke, draws a portion of the water from one of the chambers into the cylinder, lowering its level in that chamber, and permitting the air to enter through the inwardly opening valve; the return-stroke causes the water to rise, forcing some of it, together with the air,

The Athenian docks in the Piræus cost 1,000 tal"They have a design to get the king [Charles II.] to hire a dock for the herring busses to lie up in." PEPYS, 1661.

"Sir N. Crisp's project of making a great sasse [sluice or lock] in the king's lands about Deptford, to be a wett dock to hold 200 sail of ships."- IBID, 1662.

Of the docks of London :

Pitt laid the foundation-stone of the "West-India" August 15, 1800; opened in 1802. "London" docks, built 1802 - 5. "Victoria," 1855. The Liverpool and Birkenhead docks, 1810 - 57.

2. (Harness.) The divided piece forming part of the crupper, through which the horse's tail is inserted. Dock'er. A stamp for cutting and piercing dough in making crackers or sea-biscuit.

Doc'tor. A part in a machine for regulating quantity, adjusting, or feeding :

a. (Paper-making.) A steel edge on the pressureroll of a paper-machine to remove any adhering fibers. b. (Steam-engine.) An auxiliary steam-engine to feed the boiler.

c. (Calico-printing.) A scraper to remove superfluous coloring-matter from the cylinder.

The color-doctor of a calico-printing machine, which wipes superfluous color from the face of the engraved roller.

The lint-doctor, which removes fluff and loose threads from the said roller.

The cleaning-doctor, which wipes clean the surface of the roller.

Dod. (Tile-making.) A piece affording an annular throat through which clay is forced, to make drain-pipe. See TILE-MACHINE.

Dodg'ing. Said of mortises, when they are not in the same plane at the hub. By spreading the butts of the spokes where they enter the hub, dodging on each side of a median line, alternately, the wheel is stiffened against lateral strain. The wheel is said to be staggered.

Doe'skin. (Fabric.) A single width fine woolen cloth for men's wear; not twilled.

DOFFER.

Doffer. A comb or revolving card-covered cylinder in a carding-machine, which strips the fleece or sliver of fiber off the main card-wheel after the filaments have passed the series of smaller cardingrollers and the flat cards.

It is usually a comb with very fine teeth, which penetrate slightly between the wire teeth of the card as the comb moves downward.

Doff'ing-cyl'in-der. A cylinder clothed with cards which are presented in such direction and at such a rate of motion to the main card-cylinder as to remove the fibers from the teeth of the latter.

The doffing-cylinder assumes one of three forms: 1. Continuous clothing; removing a perfect fleece of the width of the machine. Such is the doffer of the scribbling-machine, which yields a continous lap or fleece.

2. Longitudinal bands of card clothing; removing slivers of a width determined by the breadth of the bands and of a length equal to that of the doffer. Such is the doffer of the slubbing-billy. See SLUB

BING-MACHINE.

3. Circumferential bands or rings of card-clothing; removing narrow, continuous slivers, which pass to the condenser, whereby they are compacted and brought to the condition of slubs. Such is the doffer of another form of SLUBBING-MACHINE (which see). Doff'ing-knife. A blade of steel toothed at its edge like a fine comb, and vertically reciprocated by a crank tangentially to the teeth of the doffer in a carding-machine, in order to remove therefrom a fine fleece of carded wool which is gathered into a sliver. See DOFFER.

1. (Pile-driving.) A grappling iron or grab, usually with jaws, and adapted to raise the monkey of a pile-driver. When the jaws open, the object is dropped or released. See PILE-DRIVER.

2. (Well-boring.) A grab for clutching well tubes or tools, in withdrawing them from bored, drilled, or driven wells. See GRAB.

3. (Turning.) A clamp fastened to a piece suspended on the centers of a lathe, and by which the rotation of the chuck or face-plate is imparted to the piece to be turned (a b, Fig. 1679).

4. A click or pallet adapted to engage the teeth of

DOG-NOSE VISE.

a ratchet-wheel, to restrain the back action. A click or pawl. See WINDLASS; RATCHET.

5. (Machinery.) a. The converging set screws which establish the bed-tool of a punching-press in direct coincidence with the punch.

b. A contrivance for holding the staff to the rest, chuck, or carriage, while being cut, sawed, planed, or drilled.

c. An adjustable stop placed in a machine to change direction of motion, as in the case of feed-motion, or in jacking, shaping, or planing machines.

6. (Hoisting and Hauling.) a. A grappling-iron (c) with a fang which is driven into an object to be raised or moved.

In the continuous system of feed in saw-mills, the chain has a number of dogs attached to different portions of its length. Dogs are also used for securing and towing floating logs and in shifting or loading logs on the ground or carriage.

6. A ring-dog or span-dog (d); two dogs shackled together by a ring, and used for hauling or hoisting. c. Sling-dogs (e); two dogs at the end of a rope and used in hoisting barrels. A span-shackle. 7. A bench-dog (ƒ) is a clamp, and holds the timber by its tusk.

8. (Sawing.) A rod on the head or tail block of a saw-mill carriage, by which the log is secured in position. The dog (g) is pivoted to the block, and its tooth is driven into the log. It varies in form on the head and tail blocks respectively.

In h and i respectively are shown other forms of the saw-mill dog. See also CIRCULAR SAW; HEAD

BLOCK.

9. (Shipbuilding.) The last detents or supports knocked away at the launching of a ship. A dogshore.

10. (Locksmithing.) A projection, tooth, tusk, or jag in a lock, acting as a detent. Especially used in tumbler-locks.

11. An andiron.

Dog and Driv'er Chuck. A chuck having two parts. The dog slips upon and is fastened by a set screw to the object to be turned. The driver is attached to the lathe-mandrel, and has a projecting arm which comes in contact with the dog, and causes it and the work to revolve with the mandrel. See DOG (a b, Fig. 1679).

Dog-bolt. The bolt of the cap-square over the trunnion of a gun.

Dog-cart. A sportsman's vehicle having shafts and two wheels, with a box beneath the seat for setters or pointers.

Dog'ger. (Nautical.) A two-masted fishingvessel with bluff bows and used on the Dogger Bank, an extensive shoal in the center of the North Sea. It is about 80 tons burden, and has a well in the middle to bring fish alive to shore.

Dog-hook. 1. A bar of iron with a bent prong to drive into a log. See DOG.

2. A wrench for unscrewing the coupling of iron boring-rods. A spanner.

Dog-leg Chis'el. A crooked-shanked chisel used in smoothing the bottoms of grooves.

Dog-legged Stairs. A flight of stairs without any well-hole, and used in confined situations. The flight goes up, winds in a semicircle, and then mounts again in a direction parallel to the first.

Dog-muz/zle. A wire cage over the nose and jaws, to keep a dog from biting; or a strap around the jaws, to keep them shut.

Dog-nail. A large nail with a projecting tooth or lug on one side; used under certain circumstances by locksmiths and carpenters.

Dog-nose Vise. (Locksmithing.) A hand-vise

with long, slender, pointed jaws. Called also pignose vise.

Dog-pow'er. A machine by which the weight of a dog in traveling in a drum or on an endless track is made to rotate a spit, or drive the dasher of a churn.

The turnspit-dogs of the last and previous centuries ran on the inside of a hollow tread-wheel, which rotated with their weight and communicated motion by a band to the spit. See ROASTING-JACK. In the modern dog-powers, as in the example, the animal walks on an endless chain-track, which slips Fig. 1680.

to the rear, rotating a drum which oscillates an arm, and vertical reciprocation is given to a lever and the churn-dasher.

Dog-shore. (Shipbuilding.) One of the two struts which hold the cradle of the ship from sliding on the slip-ways when the keel-blocks are taken out. The lower end of each dog-shore abuts against the upper end of the rib-band of the slip-way, and the upper end against the dog-cleat, which is bolted to the side of the bilge-way. Beneath each dog-shore is a small block called a trigger.

In launching, the triggers are removed, the dogshores knocked down, and the ship-cradle freed, so that, carrying the vessel, it slides down the slipways. The signal for launching is, "Down dogshores." See LAUNCH.

Dog's-tooth. A sharp steel punch used by marble-workers.

Dog-stop'per. (Nautical.) A stopper put on to the cable to enable it to be bitted, or to permit the messenger to be fleeted.

Fig. 1681.

Egyptian Doll.

Dog-vane. (Nautical.) A small vane, made of cork and feathers, placed on the weather-rail as a guide to the helmsman when sailing on a wind.

Doily. (Fabric.) Formerly, a species of woolen stuff; now, a table-napkin. Doll. A child's toy-baby. Made of stuffed cloth, wood, india-rubber, etc. The jointed wooden dolls are a marvel of cheapness, and are made by the peasantry of Europe. See Toy.

Among other curiosities of the former inhabitants of Egypt are a number of dolls which are found in the tombs, and also are represented on the painted walls. Just as with us, some are rough, some comical, and some are made as nearly symmetrical as the artist was able.

Dolly. 1. (Metallurgy.) A perforated board placed over a tub containing ore

DOME.

to be washed, and which, being worked by a winchhandle, gives a circular motion to the ore.

2. (Piling.) An extension-piece on the upper end of a pile, when the head of the latter is beyond the reach of the monkey. Otherwise called a punch. 3. A hoisting-platform.

4. A tool with an indented head for shaping the head of a rivet. A snap-head.

Dolly-bar. A block or bar in the trough of a grindstone which is lowered into the water to raise the latter against the face of the stone by displacement.

Dolly-tub. (Metallurgy.) A vertical tub in which metalliferous slimes are washed. It has a vertical shaft and vanes turned by a crank-handle, like some kinds of churns.

Dolphin. 1. (Ordnance.) One of the handles of an old-fashioned brass gun, nearly over the trunnions, and by which it is lifted.

2. (Nautical.) a. A bollard post on a quay to make hawsers fast to.

b. An anchored spar with rings, serving as a mooring-buoy.

c. A strap of plaited cordage acting as a preventer on a yard, to sustain it in case the slings are shot away.

3. (Hydraulics.) The induction-pipe of a watermain, and its cover, placed at the source of supply. Dolphin-strik'er. (Nautical.) A spar depending from the end of the bowsprit. It affords a strut for the martingales of the jib-boom and flying-jibboom.

Dome. 1. (Architecture.) A vault on a circular plan. It is usually hemispherical in form, but is susceptible of a prolonged or oblate spheroidal variation. In the data following, the hight given is that of the apex above the ground.

The dome of the Pantheon at Rome is a hemisphere 142 feet in diameter, 143 feet high above the floor of the rotunda.

The dome of St. Sophia at Constantinople is an oblate semi-spheroid 104 feet in diameter, 201 feet high. It is said to be built of earthenware and pumice-stone, not of cut stone. It was built in the sixth century.

The dome in the Duomo of Florence was built by Brunelleschi in 1417. It is of brick, octagonal in plan, 139 feet in diameter, and 310 feet in hight.

The dome of St. Peter's, at Rome, was built at the close of the sixteenth century, from designs left by Michael Angelo. It is 139 feet in diameter, 330 feet high.

The dome of St. Paul's, at London, by Sir Christopher Wren, is not masonry, but a shell inclosing the brick cone which supports the lantern. It is 112 feet in diameter, 215 feet high.

Mosque of Achmet, Constantinople

Internal Internal Diameter. Hight.

57

254

150

96

150

112

116

Duomo at Milan
Hall aux Blés, Paris, by Moulineau 200
St. Isaac's, Petersburg.
Baths of Caracalla

The dome of the Capitol, Washington, is 287 feet 11 inches above the base-line of the east front. The greatest diameter of the dome at the springing is 135 feet 5 inches. The weight of iron in the dome and tholus is 8,009,200 pounds. The rotunda is 95.5 feet in diameter, and its hight from the floor to the top of the canopy is 180.25 feet.

The central rotund of the Vienna Exposition building, 1873, springs from a circular façade of piers 426 feet, English, in diameter; within which is a gallery covered with its own roof; from the interior perimeter of the gallery rises a conical roof