SCOTUS AND SCOTISTS-SCREAMER.

disappearance is not wonderful, when we take into account, first, the strong measures resorted to, about 1530, by both civil and ecclesiastical authorities, to put down all ballads reflecting on the Roman Catholic hierarchy, and afterwards the fanatical proscription of music, along with every other innocent amusement, by the Puritans. The most valuable of now existing early collections of Scotch melodies is the Skene MS., in the Advocates' Library, noted down by Sir John Skene of Hallyards about the year 1630. It contains a number of native airs, mixed with some foreign dance-tunes-upwards of a hundred in all. Many of the Scotch melodies of any kind of metal, which are only of use for differ considerably from the more modern versions, presenting in general a ruder outline; but often exhibiting beauties which the changes which these airs have subsequently undergone have only tended to destroy.

On the subject of Scottish music generally, reference is made to Dauney's Ancient Scottish Melodies from a MS. of the Reign of King James VI., with an Introductory Inquiry Illustrative of the History of the Music of Scotland (Edin. 1838). SCOTUS AND SCOTISTS. See DUNS SCOTUS. flank of an army to observe the force and moveSCOUT, a person sent out in the front or on the ments of the enemy. He should be a keen observer, and withal fleet of foot, or well mounted.

Among the peculiarities which give its character to the music of Scotland, the most prominent is the prevalent omission of the fourth and seventh of the scale, and consequent absence of semitones, giving rise

to such melodic forms as

or

SCRAP-METAL, a term applied to fragments

remelting. Copper and brass scrap consist of the turnings from the lathe, and all useless and worn pieces, whether old or new. They are readily remelted. Scrap-tin consists of the clippings and fragments of tinned iron and worn-out tinned vessels; these are frequently dipped into hydrochloric acid, to dissolve off the tin-coating from the iron; and the muriate of tin so formed is of commercial value for dyeing purposes. Scrap-iron consists of any waste pieces of iron, although the term is usually held to mean malleable iron only; and for many purposes it is particularly valuable, as it is found that a greater strength can be obtained by welding small fragments of iron together, than is found in large masses, the fibre being much more twisted Passages of this kind and interwoven, from the mingling of pieces in every imaginable direction.

occur in all the airs of Scotland which have any claim to popularity, and form one of their most recognisable features. Another characteristic is the substitution of the descending for the ascending sixth and seventh in the minor scale, as at the beginning of the air called Adew, Dundee, in the Skene MS.

8

A very prevalent course of modulation is an alternation between the major key and its relative minor, the melody thus ever keeping true to the diatonic scale of the principal key, without the introduction of accidentals. An air will often begin in the major key, and end in the relative minor, or the reverse. The closing note is by no means necessarily the key-note, a peculiarity especially remarkable in the Highland airs, which, if in a major key, most frequently terminate in the second; if in a minor, on the seventh. Closes are also to be found on the third, fifth, and sixth. The peculiarities of modulation of the music of Scotland have something in common with the modes of ancient ecclesiastical music, to which it may be more correctly said to belong, than to the modern major and minor keys; and the avoidance of the fourth and seventh may have originated in the imperfection of the ancient wind instruments; yet these peculiarities are not to be found in the national airs of other countries where ecclesiastical music may be supposed to have had the same influence, and the early instruments to have been equally imperfect.

Among the more modern printed collections of Scottish melodies with words, the most important are George Thomson's collection, with symphonies and accompaniments by Pleyel, Kozeluch, Haydn, Beethoven, Hummel, and Weber (vols. i.-iv., 1793 -1805; vol. v. 1826; and vol. vi. 1841), one distinguishing feature of which was the appearance of Burns's words conjoined with the old melodies of the country; and a more recent collection in 3 vols., published by Messrs Wood & Co., and edited, with historical, biographical, and critical notes, by Mr G. F. Graham (1848-1849).

SCREAMER (Palamedea), a genus of birds of the order Gralle, allied to the Jacanas (q. v.). The bill is rather short, conical, curved at the extremity; there is a bare space around the eyes; the toes are long; each wing is furnished with two strong spurs. The HORNED S., or KAMICHI (P. cornuta), inhabits swamps in Brazil and Guiana, and feeds on the leaves and seeds of aquatic plants. It is of a blackish-brown colour, nearly as large as a turkey, and has somewhat the appearance of a gallinaceous bird. It receives its name from its loud and harsh cry. From the head, a little behind the bill, there rises a long, slender, movable horn, of which no use has been conjectured. The spurs of the wings

are supposed to be useful in defence against snakes and other enemies.-Closely allied to this genus is the genus Chauna, or Opistolophus, to which belongs the CHAUNA, or CRESTED S. (C. or O. chavaria), a native of Brazil and Paraguay, the head of which has no horn, but is adorned with erectile feathers. The plumage is mostly lead-coloured and blackish. The wings are armed with spurs. It is very capable of domestication, and is sometimes reared with flocks of geese and turkeys, to defend them from vultures, being a bold and powerful bird.

SCREEN, in Architecture, an enclosure or partition of wood, stone, or metal work. It is of frequent

SCREW-SCREW PINE.

SCREW-PROPELLER.

meter is the distance between the tips of opposite blades; the length is the fraction of the pitch actually employed; the slip, the loss of power from the yielding of the water.

export from Mauritius, rivalling in cheapness and blade would make one complete turn; the diausefulness the Gunny Bags of India. The leaves are cut every second year, and each plant yields enough to make two large bags. Immediately on being cut off, the leaves are split into fillets, which are nearly an inch broad at the base, but taper to a point, and are three or four feet long. One of these will support a bag of sugar, of about 140 pounds, without breaking. The aerial roots of the Vacoa are so fibrous as to be used for making paint-brushes for coarse purposes.

SCREW-PROPELLER, THE, is of the same construction as the common Screw (q. v.), but with the narrow thread exaggerated into a broad, thin

a

Fig. 1.

Ъ

The application of the screw to the propulsion of a vessel through the water is not new. In 1802, Dr Shorter, an English mechanician, produced motion by its agency; but his discovery was valueless, as the steam-engine had not then been practically applied to navigation. Those who first employed Watt's engine on board ship adopted the paddlewheel, the success of which turned attention from the screw for nearly thirty years. At length, in 1832, Mr B. Woodcroft patented a screw-propeller with an increasing pitch; and four years later, Mr F. P. Smith patented a screw making two whole turns, which he reduced, in 1839, to one whole turn. In 1837, he and Captain Ericsson, an American inventor, brought the matter practically forward on the Thames, where a small screw-steamer, 45 feet long, 8 feet broad, and of 27 inches draught, towed the Toronto of 630 tons against tide at 4 knots an hour. In 1839, an American gentleman had the Robert Stockton built for him by Messrs Laird, with which he reached America. The British Admiralty, however, refused any support to the new propeller until the success of the Archimedes, built in 1840, of 232 tons and 80 horse-power, which was exhibited at the principal ports, rendered opposition no longer possible. The Admiralty, then, as an experiment, constructed the Rattler, from the trials of which vessel nearly all the subsequent data for the screw have been derived. Meanwhile, in 1838, Mr James Lowe had shewn that the length of the screw should not exceed 4th of the pitch; and after actual and successive trials, the screw of the Rattler was cut down from 5 feet 9 inches to 1 foot 3 inches. It is obvious also that on the same spindle there to the paddle-wheel; and its advantage for ships of These experiments established the screw as a rival may be more than one blade, provided that all the war became incontestable, as, from the entire subblades have the same pitch or rate of progression mergence of the propeller, and consequent lowness along the spindle (in fig. 1, ab is the pitch of the of its engines in the ship, the chances of injury from screw). Screws have thus been formed with two, an enemy's shot were reduced almost to nothing. three, four, and six blades or arms, but the form Some of the great steam-companies-notably the most commonly used is two blades for ships-of-war, Peninsular and Oriental Company-also patronised

plate, and the cylinder diminished to a mere spindle. One complete turn of such a screw is shewn in fig, 1. Now, if a screw of this form were turned round in an unyielding substance, as wood, it would for each turn advance as much as the centre of the blade (or thread) had moved along the spindle in forming the screw, i. e., the distance ab. If the screw be fixed beneath a ship, and made to revolve in the water, the water takes the place of the wood; but as it is less stable than that substance, a certain part of the advancing power is lost, which loss is called the slip of the screw. In practice, this slip amounts to from 10 to 30 per cent.

and three blades in the merchant-service.

ab

If the screw be cut off before attaining the length ab of a whole convolution, as at c, the portion ac will still retain all the properties of the screw. In the earlier attempts, screws were tried of the length of a whole convolution, or even two whole turns; but experiment has since shewn that this length is a disadvantage, and that the maximum of power is obtained from a screw whose length is th of the pitch, i. e., where ac = A screw of this length with two blades is shewn in fig. 2, being perhaps the most common form in use. The speed of a screw in a given time is equal to the pitch multiplied by the number of revolutions, and the product diminished by the slip.

The following are the technical terms applied to the screw-propeller: The shaft or spindle is the cylindrical axis on which the screw revolves, and is the medium for communicating to it the power of the steam-engine; the blade is the thread of the screw; the pitch, the length of shaft on which the

Fig. 2.

it, and it was found of great value as an auxiliary in (1871), its use in the British navy is almost universal, sailing-vessels. The result is that, at this time and in the merchant-service few large vessels are built which have not a screw, at least as an auxiliary.

Several varieties of screw have been introduced, each finding many supporters. Perhaps the most which is shewn in efficient, under all circumstances, is Griffith's, fig. 3. Here the blades, in place of rising from the shaft itself, spring from a hollow sphere occupying one-third the screw's diameter. This arrangement was adopted because experiment proved that the central portions of the blades of the ordinary screw absorb about 20 per cent. of the propelling power, while they produce little useful effect, from the circumstance, that at that part (especially in screws of a coarse pitch) the blade is nearly in a line with the shaft, and acts at right angles on the water, causing only a disturbance of that portion on which the outer and more powerful end of the blade operates. The globe, on the other hand, revolves with little

Fig. 3.

SCREW-PROPELLER-SCREW-WRENCH

It has

friction. The blades are also less liable to be deprives them of their proper dip; whereas, with wrenched off by the resistance of the water, and the vibration on board is considerably less than with the common screw. Of other screws may be mentioned Ericsson's, in which the sphere-principle of Griffiths is extended to the use of a ring of metal, with numerous short blades round its periphery. Messrs Carlsrund and Sorensen, of the Swedish navy, reverse this process, and have the blades within the periphery. Further in the year 1865 Mr Rig had effected additional improvements, by which a large proportion of the power is employed nearly parallel to the shaft. This is brought about by inserting behind the moving screw a fixed screw with the blades turned in an opposite direction.

One difficulty in the use of the screw as an auxiliary in sailing-ships is, that in a good wind, from the speed imparted by the sails, the ship sometimes outruns the screw; and the result is, that either the screw seriously impedes the sailing, or the blades are twisted off the shaft. To prevent this, various devices are resorted to. In some cases, the screw is disconnected from the shaft, and left to revolve freely; in others, as in most ships of war, it is disconnected and hoisted altogether out of the water by means of an iron framework worked above the screw in a sort of well. Messrs Maudslay have patented a 'feathering-screw,' which, by a simple apparatus, can, when the steam-power is not required, have the blades turned into a line with the ship's keel, and the screw (which must be two bladed) fastened in a vertical position. When thus treated, the screw is out of danger, and forms no impediment to the ship's progress.

ordinary management, the screw will always be
immersed. Again, rolling deprives the paddle of
much power; while pitching deprives the screw of
its proper matrix; but the balance of loss, in tem-
pestuous weather, is in favour of the screw.
been already shewn that in men-of-war the screw is
the most useful agent; and, as an additional reason,
may be adduced the clear broadside which it allows
for the guns. On the other hand, in point of com.
fort to the passengers, the advantage lies unquestion-
ably with the paddle; for the rapid revolution of
the heavy screw on a shaft extending half the ship's
length, produces a continuous and very unpleasant
vibration; while the lower position of the engines
and screw give the vessel a deep roll. The paddles
in the paddle-steamers act as outriggers, and the
centre of gravity being, from the position of the
engines, higher (as will be understood from the out-
line in fig. 4), a much softer motion is imparted to

the whole vessel. For lakes and rivers, where the water is smooth and the voyage short, paddles are best, and more especially so when the water becomes often shallow or is choked with weeds, which would soon clog the screw.

making its way through wood.

the motion of the ship is caused by the pressure of the They assert that solid water from behind the screw, while the action of the screw creates a void in its front. In proof of this, experiment has shewn that if a disc be placed before the screw and the screw still worked forwards, the ship moves back wards, while, with the same motion of the screw, a disc behind it causes the vessel to move forward. Others, however, think that this effect of the disc can be sufficiently accounted for on the usual theory of the screw's action.

It is proper to observe that some eminent engineers deny that the action of the screw-propeller The usual position for the screw is in the dead-is to be assimilated to that of an ordinary screw wood immediately before the stern-post, the shaft on which it revolves passing, parallel to the keel, into the engine-room. To give it full effect, the stern lines should be very fine, so that the water has a full sweep over it. In 1860, Messrs Dudgeon of London produced a steamer with twin screws, one under each quarter. These have independent action, and as one can therefore be reversed while the other goes ahead, great steering-power is imparted; so much so, that vessels constructed on this principle are said to be able to turn in their own length. In the cigar-shaped yacht built at Millwall in 1865 for Mr Winans, of Baltimore, there was a screw-propeller at each end. As the action of the screw depends on the comparative immobility of the water in which it acts, it is necessary, for the development of its full power, that it should be completely immersed, and that there should be nearly two feet of water above the top of the upper blade. It follows from this that, ceteris paribus, the screw-vessel will draw more water than the paddle-steamer; for in large steamers the screw is from 15 to 18 feet in diameter, and in the Great Eastern it reaches 24. The revolutions vary from 50 to 72 per minute.

SCREW-WRENCH, a tool used for grasping the flat sides of the heads of large screws, such as are used in engines and other large works. The heads

Fig. 1.

are usually octagonal laterally, and the wrench is made of two portions like hammers sliding one upon the other (fig. 1), so that screw-heads can be grasped of different sizes, and the handle forms the lever

It now only remains to notice the comparative advantages of the paddle and screw. Under favourable circumstances, in ships of equal tonnage and power, there is little difference in speed or force. Before the wind, the paddle has a slight advantage; with the wind ahead, the resistance offered by the paddle-boxes transfers the advantage to the screw. Fastened stern to stern, the screwship drags the paddle-ship; but fastened bow to bow, the same result is not found. This is, however, rather to be attributed to the loss of power in a paddle-ship when not in progress (sce PADDLE WHEEL), than to any actual superiority of screw. a long voyage, however, the gain is distinctly with by which they are turned round. The screw-key the screw; because the weight of fuel borne at (fig. 2) is only a more simple kind of wrench, which starting sinks the paddles too low in the water, and will only act upon screws of two sizes, fitting the probably its exhaustion at the end of the voyage jaws at each end.

In

Fig. 2.

571

SCRIBBLER-SCROFULA.