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through which the rain water percolates to the chalk, in which, being upheld by the retentive strata below, it accumulates until it finds vent by several deep valleys, which incline southward, and carry off a large quantity of water by the streams Ver, Gade, Bulbourne, and Chess, into the river Colne, which runs in a S.W. direction under the escarpment formed by the outcrop of the London and plastic clays.

The surface of this reservoir or the water level, regulated by these vents, dipping towards the south at an average inclination of nearly 300 feet in fourteen miles, may be represented by a line drawn from the upper district at that angle, and terminating at the river Colne.

The southern portion is almost entirely covered by the London and plastic clays, from the surface of which the rain water flows in open drains and water-courses. A considerable portion of that which flows towards the Colne, sinks into the subjacent chalk, when it arrives at the outcrop of the sand of the plastic clay formation, and assists in the replenishment of that portion of the reservoir that underlies the London and plastic clays. Here the water level, or the height to which it would rise through perforations in these clays, where its continuity is interrupted by them, would be represented by a line drawn from the Colne to mean tide level in the Thames below London, the only apparent vent for their waters. In the upper district, during the replenishment of the reservoir, which usually occurs between December and March inclusive, the water accumulates in a proportion increasing with the distance from the river or vent, and falls off in a corresponding ratio during its periodical exhaustion, which usually takes place between April ånd November. This alternation of level, which in the upper districts exceeds 50 feet in perpendicular height, would be represented by a line fixed at the river or vent, and rising at an angle proportionate with the increase furthest from it, the extent of its rise or fall being determined by the quantity of rain percolating the chalk. The ratio between these extreme points, is so exactly maintained, that if the distance of rise or fall in two wells, one near, and another at a distance from the vent, be ascertained, the alternation in the intermediate wells will be determined with considerable accuracy.

The progressive rise of the water level is apparent at the sources of the streams which break out at higher levels in the valleys in which they run, or when brooks or burns burst forth and

VOL. XXII.

run during a certain period, when the surface of the reservoir attains a certain level, previously to which, the water rises in every depression till it reaches the height at which it can flow away; the converse of the effects, which preceded their bursting, may be seen as they cease to flow.

When no water percolates the surface of the upper district, the flooding of the Colne by heavy rains, together with the sinking of the water into the chalk, at the outcrop of the sand of the plastic clay formation, raises the level in that locality, and, by checking the drainage, retards the exhaustion of the reservoir. When this occurs during the replenishment, and from continued rain, the level near the river maintains an increased elevation, the water checked in its course towards its vent accumulates in a ratio increasing with its distance from it, a process of adjustment to be traced throughout the district during the replenishment, and conversely during the exhaustion of the reservoir.

The geological condition of the lower portion of the district, together with the paucity of wells, make it difficult to ascertain the extent of the natural alternations of that part of the water level which underlies the London and plastic clays; the difficulty is increased by an unnatural depression caused by the exhaustion of water under London, which is said to increase yearly, and indicates that the rapidity of the demand exceeds that of the supply; the alternation at that point may be from 2 to 4 feet, and is coincident with the rising and falling of the levels in the upper district.

If water be discharged from a shaft in the chalk by a power not capable of exhausting it entirely, the rapidity of the reduction of the level will gradually decrease, until it is exactly balanced by that of the supply; when the exhaustion ceases, the level will rise in the inverted ratio of its reduction; if the level be measured in a line from the point of exhaustion, a similar reduction will be found, falling off at an angle decreasing with the distance from it.

The aggregate discharge of water from under London, produces a similar effect daily measurements in one well, confirmed by some coincident measurements in another more than a mile distant, show that, beginning on Monday, the level is gradually reduced during the week; the cessation of pumping on Sunday is marked by the rising of the level by Monday morning; if any great quantity of rain falls, a sudden rise or check in the periodical fall

takes place; the resumption of any extensive or continued discharge of water may be traced; a general coincidence of rise and fall in different wells is apparent; holiday times, such as Chris mas, Easter, and Whitsuntide, may be distinguished. Thus, th measurement of a chalk well in London would show the days of the week and the great festivals, by the daily variations; the seasons would be indicated by the average difference in the height of the level at different periods of the year; and the changes of the weather by the falling of rain, would also be shown.

This

The chalk under London is of a much closer nature than that in the upper districts; it yields the water sparingly but steadily from orifices beneath those beds or bands of flint, which are the most unbroken and the strongest, and from faults and cracks which are frequently met with. The constant and increasing demand, not only depresses the level under London, but must accelerate the exhaustion of the reservoir above. When the water level, near the Colne, is suddenly raised by heavy rains, a simultaneous effect is produced on the chalk wells in London. suggests the possibility of connecting a periodical defalcation of water, observed in that stream, and the river Lea, on Monday, at those seasons when the water is short, with the exhaustion of water under London. The courses of both these streams is somewhat similar, with reference to that place, though they flow in opposite directions. There is some evidence in favour of this supposition, which may be established when more information is obtained, which will be best effected by keeping registers of the daily variations of wells in different localities, and determining the height at which the water stands, with reference to Trinity high-water mark. Such information, if brought together, would well repay the labour bestowed on its collection.

The author's views were illustrated by a number of sketches and enlarged diagrams of the geological formation of the district, &c.

Dr. Buckland was desirous to bring the subject of Mr. Clutterbuck's paper before the Institution, because he was impressed with the value of a systematic series of observations upon a matter so intimately connected with engineering as the theory of the causes of the supply of water to springs and rivers, and the rise of water in Artesian wells.

In his Bridgewater Treatise, pl. 68 and pl.69, he had illustrated, by diagrams, the causes of the accumulation of a subterranean reservoir, consisting of sheets of water diffused through strata of gravel, sand, and chalk, within the basin of London, and of the rise of water in wells and small perforations through the London clay, under the influence of hydrostatic pressure.

Mr. Clutterbuck's observations and experiments confirmed the general opinion as to the existence of these subterranean sheets of water in the chalk basin, and indicated a connection between their distant parts, by the sympathy he had observed between the sudden floods at Watford and certain wells in London, the level of which had been carefully observed, and found to rise a few hours after the occurrence of the floods at Watford. In London also, he had confirmed observations already made upon deep wells, at considerable distances from one another, and found that any large quantity of water, taken from one well, reduced the level of those adjacent.

It had been questioned whether the communication between wells of this kind took place solely through the medium of large cracks and fissures, or whether the entire masses of the permeable strata, beneath the level of the lowest springs which flowed from them, had all their pores and minutest interstices so entirely filled with water, that any abstraction of this fluid from one well was more or less rapidly replaced by a general flow towards it from every part of the water-logged stratum of sand, or gravel, or stone, in which it was excavated; on the latter hypothesis, during such a flow, the surrounding wells would be affected in the direct ratio of their proximity to that from which large quantities of water were taken.

It had been found at Brentford, that, as the number of Artesian wells increased, the force and quantity of each became diminished; a similar effect followed in the case of adjacent wells in London; the inference he would draw, therefore, was, that a very extensive supply for the metropolis could not be obtained from deep wells of this kind, although a few wells might be supplied abundantly.

The district called the London Basin, is made up of a continuous seam of chalk, from 300 feet to 500 feet in thickness, which, on the S.E. of the Colne, is covered with beds of sand and gravel, alternating with plastic clay, and over all these a thick covering of London clay, whilst the country, N. and N.W.

of the Colne, is for the most part composed of naked chalk. Beneath the whole chalk basin lies a sub-stratum of clay or gault, which is impermeable by water, and upholds the reservoir in question. The valleys in this chalk are traversed by the rivers Ver, Gade, and Chess, whose chief perennial supply of water is from springs that issue out of the chalk; in one of these valleys Mr. Dickinson had proved, by experiments made with Dalton's rain-gauge, (which, being buried 3 feet beneath the surface, received only such water as descended more than that depth,) that during about two-thirds of the year the rain which fell rarely sank 3 feet into the earth, but in November, December, January, and February, it passed down into the subjacent chalk, in proportions which accorded so constantly with the greater or less amount of rain falling in these four wet months, that he had been accustomed to regulate the amount of orders undertaken to be executed in his paper-mills, during the following spring and summer, by the indications on this rain-gauge, of the quantity of water that descended more than 3 feet in the preceding winter.

The Colne is often flooded by the effect of sudden rain which is retained upon the surface of the London clay; but that portion of its water, which is derived from perennial springs, is supplied from the overflowings of the natural reservoirs, or subterranean sheets of water which fill the interstices of the chalk, and also of the sand and gravel beds of the plastic clay formation. The surface of this reservoir is marked by the outbreak of a succession of springs, at levels gradually rising as they are nearer to the upper regions of the chalk; and as the entire supply of this subterranean stock of water is derived from rain that falls on the surface of permeable strata within the London basin, the abstraction of water from any part of this reservoir would, Dr. Buckland conceived, diminish the quantity remaining to be discharged, by springs, into rivers in the vicinity of such abstraction, by the total amount of water so transferred to any other than its natural channels.

It was asserted that the surface of the water in this subterranean reservoir did not maintain a horizontal level, but that it rose nearly 300 feet in fourteen miles, between the town of Watford and the highest spring that issued from the neighbouring chalk hills. The molecular attraction of the particles of chalk through which this sheet of water is diffused, and the obstruction presented by friction to its descent through the numerous pores

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