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globe. I took two air jars that were open above, but with contracted apertures; one of these was inverted over an inflamed jet of hydrogen, so as to form a lamp or bell glass about it: there was no effect of sound, because the downward currents from above interfered with the stream of air issuing up from beneath, and made it irregular; but placing the second receiver on the first, applying them edge to edge; so as to preserve the current of air upwards from disturbing forces, the sounds were immediately produced: and, lastly, I succeeded in obtaining the tones by the draught of a common chimney; for, by attaching a large inverted air jar to the end of a funnel pipe that came from the flue, closing the other lower opening into it, and introducing an inflamed jet of hydrogen within the lower contracted orifice of the glass, the sounds were produced.
- That the same sounds may be obtained by means different to those above described, though depending on the same cause, is shewn by some experiments made by Sir H. Davy, in his first researches on the miners' safety lamp. Small wiregauze safety lamps being introduced into air jars filled with explosive atmospheres, the gases burnt on the inside of the cylinder, and produced sounds similar to those obtained from a jet of flame in a tube.
Having thus endeavoured to account for the phenomenon of sounds produced by jets of flame in tubes and other vessels, I shall notice shortly the combustible bodies I have tried. Carbonic oxide, olefiant gas, light hydrocarbonate, coal gas, sulphuretted hydrogen, and arsenuretted hydrogen were burned at the end of a long narrow brass tube rising up from a tranferring jar placed under pressure in a pneumatic trough. Ether was burned from the end of a tube fixed in a flask containing a small quantity which was heated; but a better method, and one I.afterwards adopted, is to pour a little ether into a bladder, and then force common air in; so much ether rises in vapour as to prevent the mixture being detonating, and it may be pressed out, and burnt at the end of a tube. All these were very successful. Alcohol was more difficult to
manage from being less volatile; but it succeeded when raised in vapour from a flask and burnt at a tube. In trials made with a wax taper, no distinct tone could be produced; but when the tube was made very hot, so as to assist the current through it, something like the commencement of a sound was heard at the moment the taper was blown out by the current.
Hydrogen is by far the best substance by which to produce these tones; and its superiority depends upon the low temperature at which it inflames, the intense heat it produces in combustion, and the small quantity of oxygen that a given bulk of it requires. It is in consequence less easily extinguished by the current than other gases, the current formed is more powerful and rapid, and an explosive mixture is sooner made. With gases producing little heat by combustion, and therefore occasioning but a feeble current, the effect is increased by first heating the tube at a fire, and when not heated previously, the tone is perceived to improve as the tube becomes hot from the flame playing in it.
Some variations of the form of the vessel inclosing the flame, and the material used, have been mentioned. Globes from seven to two inches in diameter, with short necks, give very low tones: bottles, Florence flasks, and phials have always succeeded air jars from four inches diameter to a very small size, may be used. I constructed some angular tubes of long narrow slips of glass and wood, placing three or four together, so as to form a triangular or square tube, tying them round with packthread, and easily obtained tones from hydrogen by means of them; and it is evident that variations of the channel, the use of which is to form and direct the current of air, may be made without end.
May 11th, 1818.
ART. XI. On the Aqueduct of Alcantara. By George Rennie, Jun. Esq.
In tracing the origin or the progress of this undertaking, it
might be deemed necessary to investigate the moral and political institutions of the people with whom it originated; that the advancement of civilization, and its connection with the sciences, should be contrasted with the systems of social order which have successively arisen, and that the principles which conduce to the display of human knowledge (in these branches), should be severally developed; but the subject is at once intricate and irrelevant, and the difficulty of accommodating historical inferences to existing facts, is a sufficient obstacle to the present inquiry.
The annals of the Portuguese nation present throughout a series of strange, and important events. Conquests and commerce, revolutions and religion, characterise the history of a people distinguished for energy and enterprise, which, while it spread the renown of its heroes and its navigators, led the way to that species of infatuation which eventually terminates in moral debasement.
Before the reign of John V. King of Portugal, the public mind, hitherto absorbed in its foreign relations, seemed to have lost every taste for the cultivation of those arts which constitute the embellishment of a state; and it is only from the exertions of that prince, that the revival of learning in Portugal can be dated. The wisdom of his legislation, and the magnificence of his public works, entitle John V. to the appellation of Great; though it is to be regretted it was ever tainted with the association of bigotry. It was in this æra that the Aqueduct of Alcantara arose.
The situation of Lisbon, on an assemblage of hard and calcareous hills, precludes the advantage possessed by other capitals, of a ready supply of water. It occasionally exhibits itself in sining wells, but of an indifferent quality, and unfit for domestic purposes; hence it is principally used in irrigation.
It is then lifted from a well of about twenty feet deep, and ten feet diameter, by means of a nora, which consists of a chain of revolving earthen buckets of about one English gallon each. There are often two series of these resting on vertical wheels, whilst a horizontal wheel and spindle (working between them), is turned by a lever of about ten feet six inches in length, which connects an ox, or a mule, to a yoke. One of these animals, raises about fifty buckets per minute, which discharge themselves in rotation into an inclined trough above, whence it is conveyed by narrow cuts to the different parts of the quinta; a bell is usually attached to the lever to indicate that the animal is going; and thus with this rude machine he works throughout the day.
In the year 1511, the first idea of supplying the capital with water was suggested by Emanuel the Great, King of Portugal, It was proposed by his architect, Francisco de Olhando, that some neighbouring springs should be conducted to a magnificent fountain, to be erected in the Praça do Rocio, in which a column surmounted by an allegorical figure of the city of Lisbon, was guarded at the base by four elephants spouting water into a marble basin; but the scheme was unaccountably abandoned. The next attempt was made by the Infante Dom Luis, but with as little success.
According to Luis Marinho, a sum of 600,000 crusadoes was raised by public subscription to defray the expense; but the money was foolishly lavished in honour of the entry of Philip III. of Spain, into Lisbon. Five successive sovereigns passed over, until the year 1713, when the foundation stone of the present aqueduct was laid under the auspices of John V. by a Neapolitan, named Canavarro, who died during the progress of the work. He was then succeeded by Brigadier Mansel de Maya, who finished it August 6th, 1732; others say 1738; since which it has remained, notwithstanding the dreadful Earthquake of 1755, an unshaken monument of stability and grandeur, to the contemplation of present and future ages.
The writers who have hitherto given a description of this undertaking, have principally confined themselves to private
memorials; for, with the exception of Murphy, and several engravings, the public are in possession of few details, or exact
In 1749, a perspective representation was published in London, by Bowles.
In 1763, Mr. Andrew Frazer made some private notes of it. In 1768, a sketch of the grand arch was presented by Col. Elsden to Sir Francis Gosling, and on the back of it part of another one taken by Mr. Robert Gosling; and about the same time a very imperfect representation by Mr. J. Hunter. Col Elsden's, which is the best hitherto made, is now in possession of W. Mylne, Esq., who had the goodness to lend it.
In 1769, an original of this plan was presented to the celebrated Marquis de Pombal, and is now preserved in the family house at Oeras: the following description is annexed to it.
Esta nobre e suberba obra tem a seu principio no fim daRibeira de Carengue, e vay em partes por baixo da terra, e em outras partes, com arcos magnificos, pela distancia de tres leguas e meya, ou 9000 toises, tendo 127 arcos.
Este risco he aquelle pequeo porcao de este grande obra que a travessa a ribeira de Alcantara sao 35 arcos.
O arco grande tem 315 palmos de altura, 150 de largo. Estes arcos tem de distancia 400 toises.
Por Luis L'Huylier de Rozierres, Adjutant Ingénieur Volontaire sous la direction de Mons. le Lieut.-Col. Guilleaume Elsden, August 12th 1770.
Besides the information which Mr. Bell, of Lisbon, kindly afforded from the numerous documents and actual measurements possessed and made by him, Murphy, as a public writer, is also referred to, although his account is far from being satisfactory. In the Voyage fait aux Côtes de Portugal, par M
* The Portuguese varra of 5 palmos de Craveira, or 40 inches, is exactly 43 English inches, or 8,64 English inches to the palmo, consequently the great arch being 315 palmos= 229 ft. 8 in.; Span, 150 palmos=108 ft.