expense? What is the expense of annual repairs and attendance?

As for navigable canals in particular, it is necessary to ascertain the kind and the number of boats which navigate them, with their tonnage and the number of persons employed on them, and the number of horses or oxen used for towing, if they are not sailing boats, which is sometimes the case. (Egypt.) What are the towns, halting places, markets, and wharfs, &c., on their banks? What is the nature of the traffic carried on by means of the canal, the annual quantity and value of the goods transported, and their respective ulterior destination, the rate of going, and the freight for different kinds of merchandise, at different seasons? Is the canal navigable the whole year round, and, if not, what causes the impediment, is it the ice of winter or the want of water in the summer? For how long is the navigation interrupted by these causes? Does the canal belong to the government, to private persons, or to a company? In either case what is the presumed profit of the proprietors? What may be remarked as peculiarly praiseworthy or defective in the regulations of the navigation of the canal and the keeping it in repair? Do passage boats of any kind ply upon the canal, and, if so, between what places? What is the rate of travelling, the passage-money and average number of persons annually going by such conveyance? What remarkable effects have been produced, particularly along the line of the canal, by its construction; and what general effect has it had on the commerce and wealth of the country? If there are any dues paid to the state, how are they fixed and levied, and what is their annual amount?

With regard to canals for draining, besides the general observations already mentioned, the traveller should ascertain what quantity of land has been reclaimed by this means. Is it possible to ascertain the total amount of benefit conferred upon the country by such canals? What were the diseases which prevailed when the district was covered with stagnant water, and which are no longer known, or greatly diminished since the draining has been effected? As to the operations of draining, they are very important, and are effected in various ways. Drains, properly so called, are frequently covered; and often, when open, are little more than mere ditches. But of drains and draining we shall speak again. See AGRICULTURE.

Canals for irrigation, are sometimes very long, and branch

D

off in all directions; in this case, it is generally the main trunk only which deserves the name of a canal. Its several dimensions must be observed, as also the extent of land watered by its means; the source whence, and the mode by which, the water is obtained, &c.

LAKES.-Lakes are collections of water of greater or less extent, sometimes giving rise to rivers, sometimes receiving them, and sometimes situated in the direction of their course. Some lakes neither receive any rivers, nor have any issue; the former are usually small. Lakes are occasionally called seas, but not solely in consequence of their magnitude; thus the great American Lakes are not called seas, while the small Lake Asphaltite is designated by the appellation of a sea; and the Lake Aral is also called the sea of Aral: the Caspian, though a lake, is called

a sea.

It has been thought, because the Aral, the Lake of Zorrah, and some others which receive rivers while they have no outlet, are salt, that all lakes, so circumstanced, must be salt likewise; but as this is by no means generally the case, and as many salt lakes have neither affluents nor outlets, their saltness must be due to some other cause. But if, as is certainly sometimes the case, the rivers disemboguing into lakes without issue, bring into the recipient and there depose saline substances, these lakes should become annually more salt.

With regard to lakes in general, the observations to be made upon them may be comprehended under the following heads:

Name; geographical and topographical situation; height above the level of the sea, and as compared to other neighbouring lakes; form, length, breadth, circumference, surface, and depth; the nature of the bed and of the borders; the transparency, colour, temperature, and quality of the water; the affluent streams and springs; the outlets, the currents; the climate, soil, and vegetation of the basins; the height and nature of the surrounding hills, when there are any; the prevailing winds; the mean ratio of evaporation, compared with the quantity of water supplied; and any particular phenomena; the navigation and fisheries of the lake, &c.

Name.-Note the name or names by which the Lake or any particular portion of it, is now, or was formerly, known. Geographical Situation.-This should be indicated as

comprised between certain degrees of latitude and longitude; the latter, reckoned from the meridian of Greenwich. When lakes are small, it will be sufficient to give the lat. and long. of some particular part of the lake, only stating precisely which part, as for instance the point where the principal affluent comes in, or the place of the immediate outlet, and the bearing of this point from the middle of the lake.

Topographical Situation. This refers particularly to the part of the country or province where the lake may be situated, or to the region in which it may be placed; as in the hilly, woody, barren or cultivated district, in a plain, in a valley, or on a height, or its nearest distance and bearing from the sea, from some other lake or remarkable place.

Height above the Sea.-This should always be ascertained if possible, as, together with other data, it serves to explain many of the phenomena of lakes. Should there be two or more lakes in the vicinity of each other, their relative height should also be obtained.

Subterraneous Communication.-Is any such known to exist, or are there any reasons to suspect such communication, and what?

Form.-The form of lakes may be at first supposed an object not worth observing, but it has its importance; for it is evident that, unless in the case of a bed mathematically regular both as to contour and slope, every change in the quantity of water will induce a change in the contour of the lake; so that any remarkable change in this latter is a positive indication either of rise, or fall, or displacement; all of which changes are highly interesting. In lakes of very irregular form, the change in the contour generally gives the first notice of increase or diminution of the water of the lake, which in very regularly formed sheets of water, is not so soon observed. In general, the length and breadth in one, two, or more places, as the case may require, are sufficient, with the terins long, narrow, oval, round, square, &c. to specify the form. But there are cases in which a longer description of form is necessary. In some few instances, as in that of the Saïma, in Finland, the irregularity of form is such as to defy all description. Nothing, in such a case, can be perfectly satisfactory but an exact plan, and this, from the very irregularity itself, can hardly be given but by an operation requiring too much time for the traveller to devote to so special an object. When a plan cannot be

taken, travellers will do well to liken the form of the lake to that of some well known object, which he may easily do, if he can obtain a general view of it from some eminence.

Length, Breadth, and Circumference.-The line according to which the length of a lake should be taken, is not always so easily determined as might be supposed. The top or head, and the bottom of a lake, may be supposed the natural longitudinal extremities, and so they are in some cases, as in the Lake of Geneva, for example; but the question arises, what determines the top and bottom of a lake? certainly not always the point of influx of the principal affluents, and the place whence the outlet issues; for sometimes both, or one of these, as in the Baikal, will be found on the side of the lake, and sometimes very near each other relatively to its length. And again, even when the head and bottom of the lake are determined, how is the line to be measured between them? certainly not always in a straight line, for if the lake happened to be in the form of a crescent, as the Lake of Geneva just mentioned, or of an S, such a line would pass over the land. The breadth is no less a matter of difficulty than the length; for by the length is always understood the extreme longitudinal dimension; but the breadth generally means the medium width, unless otherwise expressed. Now, in irregularly formed lakes, the medium breadth is not easily ascertained; nor is there any rule regarding the places whence, and the direction in which, the breadths should be taken. As for the perimeter, it is in some cases easily obtained, and in others is very difficultly ascertained, either by reason of its great irregularity, or from the nature of the banks, which may be swampy, or otherwise of difficult access. But as there is altogether much uncertainty regarding the best mode of obtaining the length, breadth, and circumference of lakes, and as the methods employed are not only arbitrary, but frequently very unsatisfactory, we venture to recommend the practice indicated in the section entitled OPERAtions, (which see.)

Surface. The extent of surface of a lake is an object of considerable importance; indeed, it is principally in order to obtain this as exactly as possible that it is recommended to take the length, breadth, and circumference. The quantity of moisture given out by evaporation is cæt. par. in exact proportion to the extent of surface. Hence, knowing the quantity evaporated from a given surface, in a

given time, at particular seasons, or taking the mean of the year, the quantity evaporated from the whole lake, its surface being known, is easily estimated. For the method of ascertaining the ratio of evaporation, see OPERATIONS.

It is, therefore, necessary to observe what is the extent of surface of the lake in square measure. Is that extent subject to annual variation; and, if so, to what extent, and at what times of the year? To what cause or causes is such variation to be attributed? Has the superficial extent of the lake been observed to increase or decrease regularly, independant of periodical changes, for any number of years? If it be on the decrease, can any probable estimate be made of the time required for the total annihilation of the lake? What are the annual variations in the level of the surface?

The horizontality of the surface also should, if possible, be ascertained; for although the surface be generally horizontal in stagnant water, it is not always so in lakes having an outlet, the water at the latter point, or in some other part, being sometimes considerably lower than at the point of confluence of the principal affluent. The slope is sometimes. continued the whole way, and sometimes the depression extends only to a certain extent back from the outlet. surface of the slope is generally a curve.

The

Depth.-What is the depth of the lake at various places, and its medium depth? It is particularly recommended to take, if possible, correct soundings over the space immediately in front of the various affluents, in order to ascertain the extent of their alluvial deposits at the time, so that by a comparison with this, and what it may be at some future period, an estimation may be made of the quantity of sedementary matter annually brought into the lake.

Nature of the Bed and Borders.-The form of the bed, in as much as it is regular or uneven, is ascertained by the soundings, which also indicate whether the deepest part is in the middle or on either side. The sounding lead will also indicate the nature of the bed, as rock, gravel, sand, clay, mud, &c.

What is the form, and character of the borders? Are they high and steep, or sloping, or shelving; are they rocky, and, if the rock be stratified, what are the strike and dip of the strata? Are they of indurated clay, or earth, or chalk, &c.? and if abrupt, are they, as is generally the case, subject to falling in, in consequence of the erosion at the water-line? Is the quantity of land thus annually destroyed