tionally and decimally. The scales of small harbor charts vary from 1: 5,000 to 1: 60,000; that of charts of bays and sounds is usually 1 to 80,000, of general coast charts 1 to 400,000. In the United States engineer service the following scales are prescribed: general plans of building, 1: 120; maps of ground with horizontal curves, 1: 600; topographical maps comprising 1 mile square, 1 m. to 2 ft., or 1: 2,640; 3 miles square, 1: 5,280; between 4 and 8 miles, 1: 10,560; 9 miles square, 1: 15,840; not exceeding 24 miles square, 1:31,680; 50 miles square, 1: 63,360; 100 miles square, 1: 126,720; surveys of roads and canals, 1: 600. In the plotting of sections, as of railway cuttings, a horizontal or base line is drawn, on which are laid off the stations or distances at which levels have been taken; at these points perpendiculars or ordinates are erected, and upon them are marked the heights of ground above base, and the marks are joined by straight lines. To express rock in a cut, it is generally represented by parallel inclined lines; rivers by horizontal lines, or better colored in blue; the depth of sounding in a mud bottom by a mass of dots. Since it would be in general impossible to express the variations of the surface of the ground in the same scale as that adopted for the plan, it is usual to make the vertical scale larger than that of the horizontal lines in the proportion of 10 or 20 to 1.-Topographical features are represented as effectively by the brush and water colors as by the pen. Colors

are used conventionally. Thus, in the practice of the French military engineers, woods are represented by yellow gamboge with a very little indigo; grass land green, gamboge and indigo; cultivated land brown, lake, gamboge, and a little India ink or burnt sienna, adjoining fields being slightly varied in tint; gardens, by patches of green and brown; uncultivated land, marbled green and light brown; brush, brambles, &c., marbled green and yellow; vineyards, purple; sands, a light brown; lakes and rivers, a light blue; seas, a dark blue, with a little yellow added; roads, brown; hills greenish brown. In addition to the conventional colors, a sort of imitation of the conventional signs already explained is introduced with the brush, and shadows are almost invariably introduced. Topographical drawings receive the light, the same as architectural and mechanical drawings, from the upper left-hand corner. Hills are shaded, not as they would appear in nature, but on the conventional system of making the slopes darker in proportion to their steepness, the summit of the highest ranges being left white. Topographical drawings embrace but a small portion of surface, and are therefore plotted directly from measures; but in geographical maps, embracing at times a great extent of country, various projections are made use of to express as nearly as possible a spherical surface upon a plane. These species of projection are generally included under the head of mapping.

his dreams at will by operating upon the mind through the senses. With this view he left his knees uncovered, and dreamed that he was travelling at night in a diligence with a vivid impression of cold knees produced by the rigor of the weather. Waller relates the case of a gentleman who was a victim of terror on account of a dream, which he could never look upon except as a real occurrence. He was lying in bed, and as he imagined quite awake, when he felt the distinct impression of a hand placed upon his shoulder, which produced such a state of alarm that he durst not move in bed. The shoulder which had experienced the impression had been uncovered, and the cold to which it was exposed produced the sensation. Persons in whom one of the senses is defective frequently have their dreams modified by this circumstance. Darwin relates the case of a deaf gentleman who in his dreams always appeared to converse by means of the fingers or in writing. He never had the impression of hearing speech, and for the same reason one who has been blind from his birth never dreams of visible objects. The condition of the digestive apparatus has a very marked influence on dreams. When the functions of the digestive organs are properly performed, the dreams, if affected at all from this cause, are pleasant; if however any disturbance exists in this part of the system, the dreams are apt to be painful, usually propor- | tioned in intensity to the amount of disturbance of the alimentary canal. To this class of sensations may be referred those dreams produced by the use of opium and intoxicating drinks, which in part at least act by the impression made upon the digestive organs. Dreams induced by this cause are remarkable for the extravagance of the phantasmagoria they exhibit, frequently presenting shapes the most fugitive and fanciful. The dreamer often seems endowed with such elasticity that it appears as if he could easily mount to and float upon the clouds above him. De Quincey, in the "Confessions of an Opium Eater," has portrayed most vividly the effects of that narcotic in the production of dreams. But it does not require the aid of a narcotic as powerful as opium, or indeed anything beyond what ordinarily occurs in a state of dreaming, to create ideas of time and space apparently as incongruous as those narrated by the opium eater. The sleeper who is suddenly awakened by a loud rap does not begin and terminate his dream with this simple occurrence, but experiences a long train of events requiring hours and even days for their fulfilment, all evidently occasioned by the sound which awakens him, and concentrated within the brief space of time it occupies. A person who was suddenly aroused from sleep by a few drops of water sprinkled in his face, dreamed of the events of an entire life in which happiness and sorrow were mingled, and which finally terminated with an altercation upon the borders of a lake, into which his exasperated

companion, after a considerable struggle, succeeded in plunging him. It is evident that the association of ideas in this case which produced the lake, the altercation, and the sudden plunge, was occasioned by the water sprinkled upon the face, and the presumption is probable that the whole machinery of an entire life was due to the same cause. Dr. Abercrombie relates a similar case of a gentleman who dreamed that he had enlisted as a soldier, joined his regiment, deserted, was apprehended, carried back, tried, condemned to be shot, and led out to execution. After the usual preparations a gun was fired, and he awoke with the report to discover that the cause of his disturbance was a noise in the adjacent room. Kant says we can dream more in a minute than we can act during a day, and that the great rapidity of the train of thought in sleep is one of the principal causes why we do not always recollect what we dream. Dreams are often produced by the waking associations which precede them; and the most recent associations occur the most frequently in our dreams. So, too, dreams are often characteristic of the peculiar idiosyncrasies of the dreamers: a miser dreams of his gold, a philosopher of science, a merchant of his ventures, the musician of melody, and the lover of his mistress. Tartini, a distinguished violin player, is said to have composed his "Devil's Sonata" under the inspiration of a dream, in which the devil appeared to him and invited him to a trial of skill upon his own instrument, which he accepted, and awoke with the music of the sonata so vividly impressed upon his mind that he had no difficulty in committing it to paper. In like manner Coleridge professes to have composed his poem "Kubla Khan" in a dream. He had, he says, taken an anodyne prescribed for a slight indisposition, and fell asleep in his chair while reading in Purchas's "Pilgrimage" of a palace built by Khan Kubla. He continued for about three hours in a profound sleep, and awoke with a vivid impression that he had composed from 200 to 300 lines of verse. He at once wrote the fragment as it is now preserved. At this point he was called out to attend to some business. When he returned, after an absence of more than an hour, the poem, with the exception of a few scattered lines and images, had vanished from his memory. Instances like the above occasionally occur where the mind in a state of waking is aided by the processes carried on during sleep. Condillac, says Cabanis, often brought to a conclusion in his dreams reasonings which he had not completely worked out on retiring to bed; and Condorcet saw in dreams the final steps of a calculation which had baffled him when awake. But such cases are rare. As a general rule dreams are wanting in coherence and unsubstantial in reasoning. Nothing is more common than for the mind in dreams to blend together objects and events which could not have an associated existence in reality. We never dream of a past

event as past. The faces of friends long dead and events long past rise before the mind with all the vividness of real existence, and fail to excite surprise by their incongruity, because, says Dr. Hartley, we have no other reality to oppose to the ideas which offer themselves, whereas in the common fictions of the fancy, while we are awake, there is always a set of real external objects striking some of our senses and precluding a like mistake there. Secondly, the trains of visible ideas which occur in dreams are far more vivid than common visible ideas, and therefore may the more easily be taken for actual impressions."-The popular belief that in dreams an insight is frequently given of coming events is shared by many well informed persons, and is supposed to be corroborated by many remarkable cases. Some of the instances recorded may be explained by natural means. Franklin believed that he was instructed supernaturally in his dreams concerning the issue of current events. "He observed not," says Cabanis, "that his profound skill and rare sagacity continued to direct the action of his brain during sleep." The dream of Albumaron, the Arabian physician, to whom his lately deceased friend suggested in his sleep " a very sovereign remedy for his sore eyes," is explicable in a similar way. But there are extraordinary instances which cannot be explained by any known natural laws. Many of these are so well authenticated that they cannot be discredited, however loath we may be to accord to them a supernatural origin. The earliest mention of dreams is in the Scriptures and in the poems of Homer, in both of which a supernatural origin is generally ascribed to them. By the ancients, indeed, dreams were almost universally regarded as coming from the other world, and from both good and evil sources. A great number of instances are on record in the Greek and Latin classics of remarkable dreams. The night before the assassination of Julius Cæsar, his wife Calpurnia dreamed that her husband fell bleeding across her knees. On the night that Attila died, the emperor Marcian at Constantinople dreamed that he saw the bow of the Hunnish conqueror broken asunder. Cicero relates a story of two Arcadians, who, travelling together, arrived at Megara and went to separate lodgings, one to an inn, the other to a private house. In the course of the night the latter dreamed that his friend appeared to him and begged for help because the innkeeper was preparing to murder him. The dreamer awoke, but, not considering the matter worthy of attention, went to sleep again. A second time his friend appeared, telling him that assistance would be too late, for the murder had already been committed. The murdered person also said that his body had been put into a cart and covered with manure, and that an attempt would be made to take it out of the city the next morning. The dreamer went to the magistrates and had the cart searched, VOL. VI.-17

when the body was found and the murderer brought to justice. Dreams were even allowed to influence legislation. During the Marsic war (90 B. C.) the Roman senate ordered the temple of Juno Sospita to be rebuilt in consequence of a dream of Cæcilia Metella, the wife of the consul Appius Claudius Pulcher. Some of the fathers of the Christian church attached considerable importance to dreams. Tertullian thought they came from God as one species of prophecy, though many dreams may be attributed to the agency of demons. He believed that future honors and dignities, medical remedies, thefts, and treasures had been occasionally revealed by dreams. St. Augustine relates a dream by which Gennadius, a Carthaginian physician, was convinced of the immortality of the soul, by the apparition to him in his sleep of a young man, who reasoned with him on the subject, and argued that as he could see when his bodily eyes were closed in sleep, so he would find that when his bodily senses were extinct in death he would see and hear and feel with the senses of his spirit.-Aristotle wrote a treatise on dreams (IIɛpí 'Evvπvíwv), as did also Artemidorus and Astrampsychus. Of late works, Dr. W. B. Carpenter's "Physiology" may be consulted with advantage; also Maury's Le sommeil et les rêves (Paris, 1861); Brierre de Boismont's Des hallucinations, ou histoire raisonnée des apparitions, des visions, des songes, de l'extase, des rêves, du magnétisme et du somnambulisme (Paris, 3d ed., 1861); and Maudsley's "Physiology and Pathology of the Mind" (London, 1867).

DREBBEL, Cornelis van, a Dutch philosopher and inventor, born in Alkmaar in 1572, died in London in 1634. Of his life little is known; but his inventive genius appears to have given him a wide reputation, for it is certain that when about 30 years of age he was in receipt of a pension from the emperor Rudolph II., granted him for scientific discoveries. He was also taken into favor by the future emperor Ferdinand II., and made tutor to his son. Seditious movements about the beginning of the thirty years' war led to his arrest; and he would have been executed but for the intercession of King James I. of England, who took him under his protection. He lived in London from 1620, devoted entirely to scientific labors. He invented several philosophical instruments, among which, it is said, were the compound microscope and a thermometer consisting of a glass tube containing water connected with a bulb containing air. His contemporaries say that he displayed to King James a glass globe in which by means of the four elements he had produced perpetual motion, and that by means of machinery he imitated rain, thunder, lightning, and cold, and was able quickly to exhaust a river or lake. He discovered a bright scarlet dye for woollens and silks, which was introduced into France by the founders of the Gobelins manufactures.

Drebbel left two treatises, which appeared first in Dutch (Leyden, 1608); afterward in Latin, under the title Tractatus duo: De Natura Elementorum; De Quinta Essentia (Hamburg, 1621); and in French (Paris, 1673).

DREDGING, the process of excavating the sediment that collects in harbors and channels; the term is also applied to the scooping up of oysters, or anything else, from the bottom. The drainage waters and even the ebb tide have sometimes been held back by floodgates, and the waters at last let out have rushed with great violence through the channels, sweeping forward the materials that obstructed them. This is the principle of flashing or flushing applied to sewers, and is the most efficient mode of dredging. In some of the harbors in England scouring basins have been constructed especially for this purpose, as at Ramsgate and Dover. To loosen the sediment, the Dutch long since contrived a floating frame to which bars were attached, that went down to the bottom and stirred up the mud, as the machine moved along with the current. These are perhaps the oldest dredg

FIG. 1.

ing machines. The dredging machine now generally used for deepening channels and harbors is an endless chain with scoop buckets, placed in a frame which may be raised or lowered through a well in the middle of the scow upon which the apparatus and the machinery for moving it are placed. The diagram, fig. 1, will afford an idea of the general principles of its construction and use. The mud may be received in a shoot, and conveyed to an adjoining scow, or the frame may be made long enough to reach beyond the side of the vessel, and discharge itself without the intervention of the shoot. Another kind of dredging machine, which is used in soft bottoms, especially where old piles remain that would interfere with the working of the endless chain and scoops, is shown in fig. 2. It is a very efficient form, sinking into the mud and filling itself readily, and is strong enough to draw old piles from their beds. The same form of dredge on a smaller scale, usually worked by hand by means of a windlass, although sometimes by steam power, is used for

heads and long wooden handles, with their teeth turned toward each other, and uniting them by a hinge like that in a blacksmith's tongs or a pair of shears.

DREDGING, Deep-Sea, an operation much resorted to by modern naturalists to investigate the bottom of the sea and its inhabitants. It has added vastly to our knowledge of the animal kingdom in general, of its distribution in depth, and of the important part it plays in the formation of the superficial layers of the earth. The first naturalist who appears to have made use of the dredge was Otto Frederik Müller, a Danish zoologist of the last century, who obtained by its means a large portion of the specimens described by him and figured so beautifully by his brother in the Zoologia Danica. The description of his sacculus reticularis, as he calls it, and of the alternate joys and disappointments of the dredge, is very amusing. A small figure of the instrument is given among the allegorical ornaments of the title page, and has been reproduced in Thomson's "Depths of the Sea." That same dredge was afterward purchased by Tilesius, the naturalist who accompanied Krusenstern in his journey round the world, and after his return it was deposited in the academy of St. Petersburg. It consisted of a square frame, each side being in the shape of a scraper. A bag of netting was attached to it, and four iron arms were hinged at the corners and met in a ring to which the rope was attached. Little was heard of dredging after Müller's time until about 1838, when Dr. Robert Ball of Dublin introduced the modern dredge, commonly known as Ball's dredge. This is oblong, with only two scrapers and two arms. For dredging from a rowboat 18 in. by 5 is a convenient size. The scrapers may be