too technical for our work. Again, in April, 1840, before it could be recovered, several months were spent in excavating round it. A similar occurrence created an obstacle which impeded the work for three months, but instead of being withdrawn, the detached part was driven literally into the stratum, which happened to be gravel. At length, in February, 1841, after eight years' labor, the rods suddenly descended several yards ; they had pierced the vault of the subterranean waters of which M. Mulct had been so long in search. In the course of a few hours the water rose to the surface and discharged itself at the rate of 600,000 gallons per hour. The depth attained was six hundred and two yards. The pipe by which the water reaches the surface has recently been carried to a height, nearly on a level with the source of supply. At present the water flows into a circular iron reservoir at the top of the scaffold, and it is thence conveyed by another pipe to the ground. The water is of good quality, and well adapted for culinary and domestic purposes. There is no fear of the supply proving deficient, as it is derived from the infiltration of a surface of country nearly two hundred miles in diameter. The Artesian wells of Elbeuf, Tours, and Rouen, which were formed many years ago, flow in an invariable volume. The ancient Artesian well at Lillers, in the Pas de Calais, has for above seven centuries furnished a constant and equable supply. When the source of supply is less extensive, these wells may be subject to variations, but the probability of this may generally be foreseen by the geologist and the engineer. " The opportunity of ascertaining the temperature of the earth at great depths was not neglected during the progress of the works at Grenelle. Thermometers placed at a depth of thirty yards in the wells of the Paris Observatory invariably stand at 53° Fah- renheit. In the well at Grenelle the thermometer was 74° at a depth of four hundred and forty-two yards, and at five hundred and fifty yards it stood at 79°. The depth attained PRELIMINARY ESSAY. 69 being six hundred and two yards, the temperature of the water which rose to the surface was 81°, corroborating previous calculations on the subject. Now that the patient labor of so many years is brought to a close, the neighbors regret that it was not necessary to go to a depth of about one thousand yards for a supply, as the water would then have been at a temperature of 104°, and immediately applicable to bathing establishments and other places in which warm water is required." THE AaUEDUCT OF LISBON. The aqueduct of Lisbon has been long admired for the excellence of its construction, and, in point of magnitude, is not inferior to any similar edifice which the ancients have left us. That part of it situated in the valley of Alcantara, about a mile from Lisbon, consists of thirty-five arches, by which the water is conveyed over a deep vale, formed by two opposite mountains. The dimensions of it in the deepest part of the valley are as follows : height of the arch from the ground to the intrados, 230 feet, 10 inches ; from the vortex of the arch to the extrados, exclusive of the parapet, 9 feet, 8 inches ; from the extrados to the top of the ventilator, 23 feet, 4 inches ; making a total height from the ground of 263 feet. Span of the principal arch, 107 feet, 8 inches ; breadth of piers of the principal arches, 28 feet ; and thickness of the piers in general, 23 feet, 8 inches. The arches on each side of the principal one diminish in breadth, as the piers whereon they rest decrease in height, with the declivity of the ground ; they do not, how- ever, decrease by any regular progression, neither are the curves employed in the arches of one kind, fourteen of them being in the Gothic or pointed style ; the rest are semi-cir- cular, a variation adopted by the architect, Manuel dal Maga, from an apprehension, that making the five principal arches semi-circular, he would considerably add to the expense. In the rest of the edifice, much judgment has been displayed ; no part of it has failed, or appears to have received the least injury. This aqueduct was finished in 1738. The great earthquake, which in 1755, destroy- ed thousands of lives, in Lisbon, numerous houses, palaces, and churches, and shook all the peninsula, had no effect upon this noble structure, which continued to pour its waters into the ruined city, with the same equal flow, after, as before that great catastrophe. Over