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19 DIVING APPARATUS, Docks, ETC.-1. Steam-dredge. 2. Tow-boats. 3. Diving-beli.
8. Shoe; 9. Knife, of diving apparatus. 10. Tidal-dock. 11-19. Perspective and se
4. Diver with Denarouse's apparatus. . 5. Dry-dock (Toulon). 6. Helmet; 7. Knapsack;
20. Anchor. 21. Iron screw. ad seetional views of pneumatic caisson.
22. Handscrew. .
DIVINE SERVICE, a tenure by which the tenant was bound to do some special divine service, as to sing so many masses, to distribute a certain sum in alms, or the like. It differed from frankalmoigne (q.v.) in this, that the lord could distrain for the former, not for the latter, which, being an indefinite service, could be enforced only by a complaint to the ordinary or visitor.
DIVING. The "treasures of the deep" have at all times been the subject of much visionary exaggeration, and the accounts of the exploits of divers equally extravagant. We could name a popular school-book, still in extensive use, where children are seriously informed that the pearl-divers of the east acquire by practice the power of remaining under water from 15 to 20 minutes. Such statements are common enough in narratives of ancient date, in some of which the time is extended to two hours. It need scarcely be said that these accounts are absurd, no such endurance being possible. The more skillful divers may remain under water for two, or even three minutes; some modern accounts say four, and even six, but this is very doubtful. In a swimming and diving contest between some North American Indians and Englishmen in a London swimmingbath, one of the Indians, a renowned swimmer and diver, remained under water just one minute and a half, but a London artisan beat him by a few seconds.
In the Encyclopædia Britannica, Prof. Faraday describes an interesting fact to which his attention was directed by a gentleman connected with the Asiatic society, who, according to Prof. Faraday, was the first to make the observation. It was observed that by breathing hard for a short time, as a person does after violent exercise, the breath could then be held much longer than otherwise. Prof. Faraday found that he could only hold breath for three quarters of a minute, if he attempted it without preparation, but that after eight or ten of such forced inspirations, he could hold breath for two minutes. This he explained on the supposition that, ordinarily, a considerable quantity of carbonic acid remains in the involved passage of the lungs, but that it becomes completely expelled by the forced breathing, and its place supplied by atmospheric air. As regards the novelty of the observation, Prof. Faraday was mistaken, as the writer of this can testify, for when a boy, he and his companion bathers in the Serpentine, in Hyde Park, commonly practiced it. The Red Indian and the artisan above referred to also did the same; it is, in fact, a sort of preparation that a practiced diver would make almost instinctively. After a few deep inspirations of this kind, a sense of giddiness is felt, and it is not prudent to carry the experiment far beyond this stage, as å fit of insensibility not unlike apoplexy is apt to result.
This giddiness, which is always produced, and the possible insensibility, indicate & different explanation from that of Faraday. The mere removal of residual carbonic acid from the lungs is not sufficient to explain these; we should rather suggest that all the phenomena result from an excessive oxygenation of the blood, and a consequently accelerated circulation similar to that produced by breathing nitrous oxide. It will be easily understood, that if the blood be forced to take an excess of oxygen, a longer time should elapse before a fresh supply would become necessary—that is, before suffocation would take place; and the giddiness, flushing of the face, and the insensibility, are results to be expected from such an excess.
Most divers suffer severely from the continual efforts in holding the breath; bloodshot eyes and spitting of blood are common among them. This rude mode of diving is now but little used except for pearl and sponge fishing; and even for these purposes, only an uncivilized people, with very little capital and knowledge, would continue to use it, as the modern applications of science afford such immense advantages for all kinds of subaqueous operations, as will be seen by the next article.
DIVING-BELL. From what has been stated in the preceding article DIVING, it will be at once understood that for all such purposes as subaqueous works upon the foundations of piers, bridges, etc., or the exploration and raising of sunken vessels, the efforts of the unaided diver would be almost valueless, and accordingly various contrivances for supplying air to the diver have been made. The cacabus aquaticus, or aquatic kettle, described by Taisnier as having been used by two Greeks in Spain, at Toledo, in 1538, in the presence of the emperor Charles V. and a multitude of spectators, is one of the earliest reliable accounts of a diving-bell. From his description, this must have been similar in principle and construction to the modern diving-bell, but of clumsy dimensions, and wanting in efficient means of renewing the supply of air. Dr. Halley's diving-bell, about 1720, was a wooden chamber of about 60 ft. internal capacity, open at the bottom, where it was loaded with lead to keep it perpendicular in its descent. Strong pieces of glass were set in the upper part to admit light. Casks filled with air, and loaded with lead, were let down with the bung-hole downwards; and from these a supply of air was drawn by means of a hose. The form of diving-bell now in use was first constructed by Smeaton for the works at Ramsgate harbor, 1788. It was of castiron, and weighed 50 cwt.; its height, 44 ft; length, the same; and width, 3 feet. It sunk by its own weight, and was lighted by stout pieces of bull's-eye glass firmly cemented bý brass rings near the top. The principle of the diving-bell will be easily understood by floating a piece of lighted candle or a wax-match on a cork, and then covering it with an inverted tumbler, and pressing downwards; the candle will descend below the level of the surrounding water, and continue burning for a short time, although the tum