.: 



our A] 



CANAL. 





desirable, in order to allow it to accommodate certain local sources of 

 traffic; but after due allowance has been made for these disturbing 

 causes, the physical characteristic* of the district mutt be carefully 

 attended to. Trial level* on the line of the axis, and cross section, of 

 the country, must be taken ; the various springs and water-courses 

 muat be gauged, and the actual uses to which they are applied iiiunt 

 be ascertained; and the geological characteristics of the district, both 

 upon the nufaoe and at nine oonaiderable depth, muat be ascertained ; 

 as well for the purpose of ascertaining the capabilities of the district 

 to upply the inateriaU of construction, aa for the purpose of obviating 

 any tendency on the part of the natural ground to facilitate the passage 

 of water. Very elaborate investigations muat be made into the con- 

 dition! of nippfy of water to the summit level, and that supply must 

 be proportioned exactly to the want* of the navigation ; the latter will 

 depend upon the length of canal to be supplied by the summit or 

 regulating bay, and will be influenced by the length, breadth, and 

 blight of the "locks, and by the nuinU-r of boats passing. Under some 

 circumstances, when, for instance, the supply of water is liable to 

 be deficient, it may oven bo advisable to substitute inclined planes 

 for locks : in other cases, it may be necessary to compensate for the 

 irregularities in the character uf the natural water-courses, by forming 

 impounding reservoirs, to which the surplus rainfall of the watershed 

 may be stored. It is desirable, in laying out a canal of this descrip- 

 tion especially, that the reaches of level channel should be as long as 

 possible in order to admit of a concentration of lockage ; for by that 

 means an economy will be effected in the time nf passing boats through 

 the locks, and in the superintendence and repairs of those works. 

 In some very exceptional cases, the summit of the depression, or 

 iv, between the head* of two valleys is occupied by a small mountain 

 _ ke, which pours its waters respectively into the natural channels of 

 either side, and thus forms at once a regulating level, or roach, in the 

 navigation. The pond of Longpendu is an illustration of this peculiar 

 geological condition, for it pours its waters into the Loire on one side 

 through the Bourbiuue, and into the Saflue on the other, through the 

 Heune, and thus acts as a natural feeder to the summit level of the 

 Cana du Centre The pond of Cony presents the same character, for 

 it communicates on one side with the Moselle, and on the other side 

 with the Saone; but this natural advantage has not yet been turned to 

 any practical account, and the only artificial navigation of this part of 

 France is effected by means of the Canal du Rhone au Rhin. In 

 South America there is also a river of some magnitude which divides, 

 in one part of its course, into two branches, one of which flows into 

 the great river, the Amazunas, and the other into the Orinoco; but the 

 occurrence of such natural facilities is very rare, and, in the majority 

 of caaes, the summits of the passe < between the heads of two valleys 

 have to be passed by means of tunnels or of deep cuttings. 



The dimensions to be given to a canal will depend upon, 1st, the 

 nature of the navigations already existing, which it is desired to place 

 in communicati >n ; and, 2ndly, the traffic likely to be afforded by the 

 locality traversed. In England it happens, from the fact of the extra- 

 ordinary distance to which the tidal action of our rivers extends, that it 

 is possible to confine the artificial navigation to very small barges; but 

 upon the Continent, in North America, and in our own East Indian 

 ilr.minionn.it would appear that there is a growing tendency to employ 

 large boats, especially upon lateral canals or improved rivers. Thus, 

 upon the Rhone the barges formerly used were about 75 tons burden. 

 but the boats latterly constructed are made to carry about 120 tons ; 

 whilst upon the Seine and the Rhine (in the lower parts of their 

 courses), barges are often used of a burden of from 400 to 500 

 tons ; and hi the United States many of the mixed canal and river 

 barges are at least of 120 tons burden. In the. execution of tin- 

 artificial inland navigation hitherto formed, very great irregularities in 

 the dimensions adopted have also prevailed in consequence of the 

 opinions entertained as to the best description of barges ; but the 

 inquiries of the French engineers seem to have led them to the con- 

 clusion that it was desirable to group canals into two classes (so far as 

 their dimensions were concerned), under the names of canals d </,;n,,/. 

 navigation sod canals d pttiti The former they made about 



32 feet 9 inches wide on the floor-line of the canal, and 47 feet 5 inulic.- 

 upon the surface or water-line, with a depth of about 5 feet 9 inches ; 





Canal da Centre. 



of 5 feet; but in some of our mineral districts the width it area 

 reduced to 16 feet, and the Urgent of these secondary canals rarely 



Chesapeake to Delaware. 



attains the width of 28 feet. A similar.irregnlarity in the dimensions 

 if canals exists in America, for the St. Lawrence canal has a width uf 



the lock* being made 106 feet 8 inches long, by about 17 feet wide, and 

 the towing-paths 13 feet wide. The canals d pttite narigati'm nn 

 made only 85 feet wide on the water-line, with a depth of 5 feet, 

 sad the locks ere constructed of 100 feet long by feet 1 inch wide 



Amai* EnflUh Canal. 



la England the dimension* adopted have varied within a very wide 

 range, or from 31 to 48 feet upuii the water-line, with au average depth 



. I 



. 





Small English Canal. 



150 feet, and the Morris canal is only 82 feet wide, on the water-line. 

 it would be desirable that greater uniformity prevailed in this respect, 

 or the inequality of dimensions often renders it impossible to pass the 



Crozat Canal. 



Barges of one canal to another ; and perhaps the dimensions odop' 

 the French engineers may, after all, be those which it would I 

 most advisable to adopt in the formation of new canals. 



When the dimensions and direction of the canal are settled, the next 

 ixjmt to be considered, as before said, is the mode of supplying the 

 water required to compensate for the various sources of loss. These 

 sources are of two descriptions, one of which is independent of the 

 traffic, and the other in direct connection with it. The former of these 

 sources of loss is produced by the evaporation from the exposed surface 

 of the canal, the nitration through the banks, the leakage at the locks 

 and lock-gates, and the loss which invariably occurs when a canal is 

 refilled after being laid dry for repairs. The second source of loss is 

 the one resulting from the consumption of water required to pass the 

 boats through the locks, and occasionally from the sudden afflu< :. 

 boats to the lower reaches of the canal. 



The evaporation from any water surface will depend, of course, upon 

 a very complicated series of phenomena, resulting from the conditions 

 of the physical geography of the district in which it is placed. The 

 laws affecting this branch of the inquiry would involve a discussion of 

 some length, and they are, therefore, reserved for the articles, 1 " 

 KATIOK and HYOBOMKTHY. For the present it may suffice to say 

 in the temperate latitudes of Northern Europe, the average ilai 

 by the evaporation from the exposed surfaces of canals and tluir 

 feeders is reckoned at from ^ to fa of au inch ; but in summer it often 

 happens that the loss may reach even (> four inches in the twenty-four 

 hours ; and, as it is precisely at this season that the ordinary c, 

 nations are deficient, it is essential to provide rather against the 

 maximum, than against the aitrayr, evaporation. As to tho loss by 

 ii. its amount must depend upon the care with which the work* 

 are executed, and the nature of the bed of the canal itself ; bir 

 when these various conditions are of the most favourable description, 

 the loss of water may range between one-half and twice the amount 

 lost by evaporation from the surface. It is usually considered that an 

 allowance of two inches per twenty-four hours is sufficient t< ' 

 pensate for any loss of this description ; but it must be observed, that 

 the first time the water is turned into a canal. <-\. n with a most 

 perfectly water-tight bed, a great quantity will be absorbed by tho 

 exposed surfaces before they become saturated, and this loss will be 

 repeated on every occasion of laying the canal dry, though of course in 

 a slighter degree than on the first occasion. It is usual to calculate 

 upon the absorption of water at the rate of four inches in twent 

 hours, until the bed shall have become thoroughly saturated. In 

 practice there is also another serious cause of loss of water upon canals, 

 arising from defects in tho execution of the locks and lock-gates ; for, 

 even when these constructions are new, they may allow a small quan- 

 lity "f water to pass, and after several years' wear they are cert 

 nflonl passage to increasing quantities, it is not rare to find the leakage 

 of an old pair of gates equal to about 10,000 cubic feet per day. 



The consumption of water for the passage of boats must <! 

 n p.. u tlu-ir numbers, upon the relative proximity of the lock 

 upon the direction in which il n tikes place. Probably it is 



Mitlii ient to provide a cu|.].]y of water to compensate for this loss, 

 calculated at the rate of 18,000 citl>i. I. et for each boat which passes 

 tin on^h tlie lock of a large canal : this quantity is no doubt in excess 

 xhieli is absolutely required, but the carelessness of working- 

 men is so great, that it is advisable to make a provision rati 

 excess, than short of, the real necessities of the case. It is custom. n\ . 

 also, to allow about & of the quantity absolutely required for the 

 lockage, to compensate for the displacement of water which sonn 

 arises from the accumulation of boats in the lower reaches of a canal. 

 The dimensions to be given to the reservoirs, or to the regulating 



