RESERVOIRS. 



RESISTANCE. 



30 



rammed ; care being taken to prevent anything like a perfect horizon- 

 tality of bedding, by making occasional steps in the layers of earth. 

 Similar precautions must be taken in the formation of masonry dams, 

 and the layers, or horizontal beds must be stepped. It is also to be 

 observed that there is a tendency in all long straight dams, of earth, 

 or of masonry, exposed to the constant action of a great weight of 

 water to assume in time the form of a catenary curve ; and in order to 

 resist this tendency it is customary to make earthwork dams segments 

 of large circles in plan, with their convex side towards the water ; and 

 in masonry dams, it is usual to form strong counterforts on the down 

 side, even when the same segmental horizontal outline, and the before- 

 quoted thicknesses have been adopted. The usual rule is, it may here 

 be added, to make the top of the clam about 5 feet above the maximum 

 water level, in order to resist the action of the waves in the reservoir ; 

 a precaution which was proved, by the accident to the dam of the 

 Croton reservoir, to be by no means exaggerated. 



One of the most important accessory works of a reservoir is the 

 bye wash, or waste weir ; a portion of the enclosure over which any 

 water is allowed to pass after the reservoir itself is filled to its maxi- 

 mum water line. The length of the bye wash must be made sufficient 

 to carry off in a thin stream, the surplus products of the most violent 

 storms, or of the most continuous winter's rains ; and it must be 

 made in such wise that the water flowing over it should not affect 

 the stability of the dam, either by the cataract of the falling water, or 

 by the velocity of the stream flowing from the foot of the bye wash. 

 For the service of canals, or of town distribution, the water is drawn 

 off by a series of sluice-gates, provided with the necessary contrivances 

 to regulate the velocity of flow in the distributing channels ; and it 

 may be stated that, for a town distribution, the variable head upon the 

 main pipes which is usually admitted, does not exceed 13 feet; and 

 that in all case* it is desirable that the efflux of the water through 

 the feeders, or the mains, should take place with as great regularity 

 as it in possible to attain. 



Some idea of the importance of the reservoirs formed for the supply 

 of canals and town distributions may be derived from the following 

 notes upon some of the most remarkable works of this description. 

 Thus the Fountain Reservoir of the Croton aqueduct covers 400 acres, 

 and has a depth at the dam of 33 feet : the Manchester Corporation 

 .vorks cover an area of above 408 acres, and have an average 

 depth of 35 feet 6 inches, or they have a storeage capacity of 584,866,716 

 cubic feet ; the Liverpool reservoirs have a storeage capacity of about 

 3,156,000,000 gallons; the Yan Yean reservoirs in Victoria, have an 

 area of 1303 acres, and a storeage capacity of 6,400,000,000 gallons. 

 The Canal du Midi, of France, has at Gros Bois, a reservoir of 

 800,510,000 cubic feet capacity, with a head of water at the dam of 

 69 feet 6 inches ; the reservoir of the Nantes to Brest canal baa a 

 capacity of about 262,395,000 cubic feet, and a head of 33 feet ; the 

 reservoir of St. Frriol, on the canal du Midi, has a capacity of 

 "1,000 cubic feet, and a head of 105 feet. The Birmingham 

 canal has a reservoir of 80 acres area, with a depth of 45 feet at the 

 bead ; the reservoir of the Union canal of Philadelphia, is not less than 

 730 acres in area, by an extreme depth of 40 feet, and it has a capacity 

 '10 cubic feet ; on the Rideau canal of Canada, there is a' 

 dam 70 feet high ; the Turton and Kntwystle reservoir, in Lancashire, 

 is of 100,000,000 cubic feet in capacity, Ac. In England, it seems 

 that the usual ratio of the storeage capacity to the area of gather- 

 ing ground is from 30 to 40 thousand cubic feet per acre ; and it is 

 found practically that the maximum quantity stored never exceeds 

 15 inches over the area of the gathering grounds, in the most favour- 

 ably inclined and impermeable districts ; nor does it ever exceed 

 he in flat retentive ones. The coat of forming large reservoirs 

 has rarely, if ever, fallen short of 45W. to 50W. per million cubic feet of 

 the storeage capacity, leaving out of account the accessory works, or 

 the purchase of the land. A very important element in the cost of all 

 such works is to be found in the compensation to be given to the 

 partiea who were formerly entitled to the water privileges existing in 

 the district ; but it is not poasible to discover any general rule appli- 

 cable to the countless variety of cases of this description. Some 

 remarks on the subject will be found under WATER MILLS. 



There is an important distinction to be made between the various 

 kinds of reservoirs used upon town distributions, which may be 

 mentioned here, though the discussion of the principles of their 

 construction must be referred to WATER SUPPLY. The distinction in 

 question is the one between the open storeage reservoirs, and the 

 covered reservoirs, from whence the waters are distributed, generally 

 after nitration. The latter are extremely costly structures, and are, 

 therefore, rarely made larger than would be required for a four or a 

 wren days' supply at the maximum ; whilst the storeage reservoirs are 

 made large enough to hold a quantity equal, when the rain-fall is 

 about 60 inches per annum, to about 120 days' consumption; when 

 out 48 inches, to about 140 days' consumption ; and when it is 

 about 22 inches (which is the case in the eastern parts <>f England), to 

 JOO clays' consumption. 



It may be interesting, archmologically, to add that some of the most 

 extraordinary mountain reservoirs in Europe are those which were 

 constructed by the Moon of Spain; that the Egyptians under the 

 Pharaohs had executed some wonderful works for the purpose of 

 storing the inundation waters of the Nile , that Ui: native rulers and 



the Mohammedan conquerors of India vied with one another iu the 

 construction of the colossal tanks of that country ; and that even the 

 Aztecs had adopted a peculiar system of aguadas, or underground 

 reservoirs, for the purpose of storing the copious but partial rains of 

 their country. The Moors and the Egyptians seem to have used the 

 water they stored principally for the purpose of irrigation. The Greeks 

 and the Romans supplied their towns with water from springs and rivers, 

 and very rarely attempted to execute irrigation works, so that there are 

 no authentic records of the formation of artificial storeage reservoirs by 

 either of those nations. One of the most ancient works of this de- 

 scription whose history can be identified in modern times is the 

 reservoir of St. Feriol, on the Canal du Midi ; and nearly contem- 

 poraneously with it the system of catchwater ponds, which supply 

 partially the fountains of Versailles, was executed by the orders o 

 Louis XIV. 



RESIDUAL, an expression which gives the remainder of a sub- 

 traction, as a b. 



RESIDUARY LEGATEE. [LEGATEE.] 



RESIGNATION. [BENEFICE.] 



RESIN. [TURPENTINE.] 



RESINEIN. [TURPENTINE.] 



RESINS, a large class of substances, existing chiefly iu the vegetable 

 kingdom, and of which common resin, rosiu, or colophony, is the type. 

 They are generally obtained by incising the bark of trees; oleo-resin, or 

 a mixture of a volatile oil and resin, then exudes, and gradually 

 hardens. It is possible that the resins do not exist as such in plants, 

 but that they are produced by the oxidation of essential oils. 



The chief properties of resins are : insolubility iu water ; non- 

 volatility ; solubility in alcohol, ether, benzole, essential oils, and by 

 the aid of heat in fixed oils ; are insulators of electricity ; become 

 negatively electric by friction ; fuse when heated ; and, in contact with 

 air, burn with a bright, but very smoky, flame. 



But little is blown of the constitution of resins. Constantly, but 

 slowly, absorbing oxygen, with or without evolution of carbonic acid 

 or water, or both, it is almost impossible to obtain them iu anything 

 like a definite state. Again, with one or two exceptions, they do 

 not crystallise, and this greatly increases the difficulty of determining 

 their individuality. Some- such as santonin, eugenin, gamboge, cube- 

 bin, myrrh, and chrysophanic acid seem to be produced by the simple 

 replacement of a number of equivalents of oxygen for an equal number 

 of equivalents of the hydrogen in the essential oil. Others, for 

 example, styracin, anime, red resin of rhubarb, mastic, amber, eleini, 

 and aloetiu ; not only take up oxygen in the place of hydrogen, but 

 als assimilate water*. Many are probably mere oxides of the volatile 

 oils ; some we know to be simply hydrates. As yet, the resins have 

 not been artificially formed from the pure essential oils : could this be 

 accomplished, an important insight into their constitution would, no 

 doubt, be obtained. 



The solutions of several of the resins in alcohol redden litmus-paper, 

 indicating that they are acid bodies. Such resins combine with alkalies 

 to form soaps, differing but little from ordinary soaps. Common 

 rosin, for example, contains tylvic add, which crystallises in prisms ; 

 pimaric acitl, which is obscurely crystalline ; and pinic acid, which is 

 amorphous. These acids, as well as many other solid and liquid bodies 

 existing in or obtained from resins, will be found described somewhat 

 more in detail under TURPENTINE. 



Many of the resins are used in medicine*; several have considerable 

 commercial value, being used for the preparation of varnishes by 

 dissolving in turpentine, wood naphtha, or spirit of wine. 



The more important resins are treated of in this division of the 

 ENGLISH CYCLOPEDIA under their respective names, or in the NATURAL 

 HISTORY DIVISION either individually or under the name of the plant 

 from which they are obtained. 



(I'ltm-rain*, as the name implies, are mixtures of GUM and resin. 



For Icicane, and resin ofWcica, see TURPENTINE. 



The resins of commerce are in some cases natural exudations, while 

 others are obtained from vegetable compounds by the action of alcohol. 

 They are for the most part brittle, tasteless or insipid, and fusible at a 

 moderate heat ; they seldom have any smell; they generally burn with 

 a strong yellow flame, emitting a large quantity of smoke. 



The ordinary resin of the shops is colophony, obtained as a residue 

 after the distillation of oil or spirit of turpentine from common tur- 

 pentine. The black resin is the cooled brittle mass in the state in 

 which it leaves the still; whereas the yellow resin is the black modified 

 by the action of water. Rain-oil U made in London, Liverpool, Bristol, 

 Hull, and Glasgow, as an ingredient in lubricating grease for railway 

 axles and for the bearings of heavy machinery. It is also used in 

 France as an ingredient in printers' ink, thereby giving the unpleasant 

 odour which is often observable in newly-printed French newspapers. 

 Most of the common resin comes from North America, to the extent of 

 20,000 tons or more annually. Resin-gat is largely made in America ; 

 but in England it does not successfully compete with coal-gas. The 

 chief uses of the various resins in England are for varnish, lacquer, 

 sealing-wax, and dyeing. 



RESISTANCE is a power by which motion, or a tendency to 

 motion, in any body is impeded or prevented. When a weight or 

 pressure acts upon a beam or bar in any direction, the tenacity by 

 which the particles of such material oppose that action constitutes a 



