stratum to keep down the water at a level below that to which it would naturally have a tendency to rise.

When the strata are thick and porous, as the sands around Bagshot, Reigate, or Woburn, a large portion of the rainwater is at once absorbed, and permeates freely through them. The infiltration is in proportion to the absence of any admixture of clay in the mass of the sands. But if, instead of this incohesive structure, the strata should be more solid and compact, then, although they will imbibe and retain a certain quantity of moisture by capillary attraction, water will not pass freely through them, except by means of cracks and fissures in the rock. Consequently such deposits (oolites, limestones, some sandstones, &c.), instead of holding definite quantities of water in proportion to their masses, will hold indefinite quantities proportioned only to the number and magnitude of the crevices and fissures by which they are traversed; and the water so held will not pass indifferently in any direction, but must follow the irregular and uncertain channels presented by these joints and fissures.

Where the water is thus diverted into comparatively few channels, the springs issuing from such rocks will frequently, where the mass of the deposit is large, be more abundant than in more permeable strata, in which the distribution of the water is more equable, and the issues more numerous. It is on this account that the springs in the chalk valleys, and at the foot of the chalk downs, are so large and powerful in comparison with those generally in the Greensand and other sandy districts.

9. But when the permeable strata not only repose upon, but are also covered by others which are impermeable, the water, which passes into the strata where they come to the surface, and follows their course underground, will be forcibly restricted within fixed limits by the superincumbent impermeable beds; and if these latter should be anywhere

pierced or bored through, then, on reaching the underlying permeable strata, the water therein will tend to rise through the opening to a height proportionate to the level at which it stands in those strata at their outcrop,—and if that should be higher than the surface of the ground where the bore has been made, the water will not only rise to the surface, but may overflow.

It is to this last-mentioned form of well that the term Artesian has been applied. The name owes its origin to the supposed fact of these wells having been originally constructed*

The construction of these wells revived, rather than originated, in Artois. They have also been long in use in Italy, and a few other parts of Europe. Shaw, in his "Travels in Barbary," mentions wells in the desert of Sahara which were evidently Artesian. That they were known also to the ancients admits of no doubt. Arago

refers to a quotation of Niebuhr, from Olympiodorus, who states that in his time (the 6th century) wells were constructed, in the oases of the desert, 200, 300, and sometimes 500 "aunes" deep, and that from these wells streams of water were thrown out, which were used for the irrigation of the country.

The most ancient Artesian wells of which I can find any record, are those mentioned in a communication to the French "Institut," by M. Lefebvre. They were discovered by M. Ayme in the Oasis of Thebes, and have there been, in places, so numerous, that the ground is, as it were, riddled (criblé) by them. They are now all neglected and in ruins; but from an examination of them by M. Ayme, it appears that a square shaft six to ten French feet on the sides was first sunk through about 80 feet of clays and marls, reaching at this depth a mass of limestone, through which the work was continued by a bore of six to eight inches in diameter at top for about 300 feet, when strata of water-bearing sands were met with. A triple layer of wood was used for lining the sides of the shafts, the decay of which has caused their stoppage by the falling in of the ground. The bore in the limestone probably did not require tubing.

So abundant seems to have been the supply, that the water from these sands not only rose through the bore in the limestone, and filled the shaft above, but appears, from the precautions taken, to have overflowed on arriving at the surface. The distant mountainous region of the kingdom of Darfur, 10o further south, is supposed to be the district in which the water-bearing strata come to the surface, and which thus furnished to the ancient Egyptians, in this nearly rainless country, an abundant and constant supply of water. The great thickness of the limestone rock, and the style of their construction, render these works very remarkable.

Ayme Bey further states that in several oases of the Libyan desert, where there are no rivers or springs, and it never rains, the population, although now very scanty,

+ Comptes rendus de l'Acad. des Sciences, Vol. VII. p. 595, 1838. Vol. XIV. p. 917, 1842.

in the county of Artois (now included in the département du Pas de Calais) in the north of France, where the geological structure of the country is singularly favourable for their easy and economical construction.* (See Appendix A.)

These wells have been in use in this part of France from a very early period. There is one, the depth of which is not known, now in action at Lillers, that is said to have been made in 1126, and never to have shown any variation in its yield of water.†

was formerly large, having been then supplied by Artesian wells, which have been allowed to fall into decay. So sound appears to have been the principle of their construction, and so well preserved are the more essential parts of these works, that Ayme Bey has cleared out and restored several of them with perfect success.

These, however, are not solitary instances. Artesian wells appear to have been common generally in the East at a very early period. The wells known as the "Wells of Solomon," in the flat and parched plain of Tyre, are, it is suggested by Lamartine ("Travels in the East") probably of this description. He states that they form three reservoirs of clear and running water, each from sixty to eighty feet in circumference, and of depth unknown. They are elevated about twenty feet above the level of the plain, and the water is perpetually running over their sides in quantity sufficient to give motion to the wheels of mills, and is still conveyed by aqueducts to Tyre. So old are these wells that the tradition is, that Solomon caused them to be constructed in return for the services rendered him by Tyre in the building of the Temple. I must, however, observe that other travellers have considered them merely as springs with walls built round them to dam in the water.

臺

For details respecting the construction of Artesian wells I would refer to M. Garnier's "Traité sur les Puits Artésiens," Paris, 1826; or for the more theoretical investigation relating to them, and for many historical details of interest, to M. le Vicomte Hericart de Thury's "Considérations Géologiques et Physiques sur la Théorie des Puits forés, ou Fontaines Artésiennes," Paris, 1826; and his "Rapport sur la concours pour le percement des Puits Forés,” etc., Paris, 1831. The best geological account of the strata traversed by these wells in France, and of the geological position of the waters, is contained in a Paper by M. D'Archiac, “Sur la Formation Crétacée,” etc., in the "Mémoires de la Société Géologique de France," 2nd Ser. Vol. II., and in the very complete special work on this subject by M. Degousée. “Guide du Sondeur, ou Traité Théorique et Pratique des Sondages," Paris, 1847. M. Arago also gave a very able notice of these wells in the "Annuaire du Bureau des Longitudes," for 1835. In this country we are almost entirely without works on the practical part of the subject, with the exception of the useful" Rudimentary Treatise on Well-digging, Boring, &c.," by Mr. J. G. Swindell: Weale, 1849. Matthew's "Hydraulia" does not treat of this question.

+ This, of course, must be very conjectural.

Of late years these wells have been extended over almost the whole of the Chalk and Tertiary districts of the north and centre of France, and have been generally found to succeed.

As the supply of water by Artesian wells in France affords several points of comparison, and is interesting on account of its bearing on the same question in this country, I have subjoined a short account of some of them, especially of those of Grenelle and Calais, in Appendix A.

II. GENERAL PHYSICAL CONDITIONS.

§ 2. On the Geology of the Country around London, with reference to the position of the Water-bearing Strata.

10. BEFORE entering upon the general question, it may be useful to give a very short description, or rather outline, of the Geology of the district under review.

London stands on Tertiary strata,-beneath which experience has proved the existence of the Chalk; and induction leads us to presume that beneath that upper member of the Cretaceous series we should find the several lower members of the same series.

With regard to the superficial areas which the different members of these two groups respectively occupy, it will be found that, commencing at a point near Hungerford, in Berkshire, and proceeding eastward, the Tertiary series gradually expand, and form a long irregular triangular area, of which the base extends from Deal in Kent to Orford in Suffolk. The Eocene or older Tertiary strata, which occupy this tract of country, consist of the three following groups.*

First. An upper group, composed chiefly of permeable, loose siliceous sands, from 300 to 400 feet thick, known as the Bagshot Sands.

Second. A middle group of impermeable, dark brown and bluish grey tenacious clays, attaining in some places a thickness of 400 to 500 feet. This deposit is termed the London

Clay.

*The Crag, being confined to Norfolk, Suffolk, and part of Essex, is omitted.