Whence it appears, that Sir John has, by these simplifications of complex formulæ, rendered an important service to practical engineers. Sir H. Douglas applies these formulæ, or rules derived from them, to the construction of a rope bridge, where the span is 130 feet, the depression 12 feet, the weight of the bridge unloaded 12,490 lb., and when loaded with infantry 34,090 lb. From which 6. The greatest strain, or that at the highest point, un loaded, 17853 7. Loaded, 48729 8. The angle which the chain makes with the vertical, at the highest point, 69° 32' Now, it appears, from tables given by Sir H. Douglas and others, that— A rope, 3 inches in diameter, or 9.42 in circumference, will sustain a weight of A rope of 6 inches in circumference, And the two suspension cables, These are the tensions which the ropes can safely support without risk; and, consequently, the floor cables alone have abundant strength to sustain all the strains to which they may be subject. The author then adds, that "One of the principal arches of Trajan's Bridge*, across the Tagus at Alcantara, having been destroyed by the French, Lord Wellington found it necessary to direct that a communication across that bridge should be reestablished, for the purpose of bringing up artillery and stores from Badajos, for the attack of the forts at Salamanca. Timber of sufficient dimensions to effect this could not be easily procured, and indeed any application of that material to make good such a fracture would have been extremely difficult, and required much labour to be performed on the spot, in fashioning, framing, and setting up the work; and which, consequently, would have given warning to the enemy, before the campaign opened, of some important movement in that quarter being intended. To obviate these difficulties and objections, (Lieutenant-Colonel Sturgeon, of the Royal Staff corps), the officer sent in An elegant plate of this bridge, repaired as here described, forms a beautiful frontispiece to the title-page of this work. April 1812 to make preparations for this operation,-a man of fertile genius and great practical knowledge, happily devised an application of cordage, which might be prepared secretly, and even in privacy, at any distance from the place at which it was to be used, and easily transported thither entire, and speedily stretched across whenever it might be required. The broken arch left void was about 100 feet wide, and between its shattered buttresses a precipitous chasm 140 feet deep. Across this abyss the slender material spread gracefully but securely. A work so slight and temporary, contrasted strangely with a massive structure which had stood for ages; but the happy expedient made good, in a few hours, a way over ruins of vast account; and formed an auspicious opening to the important scenes that were now about to be acted on the great theatre of war." The seventh and last section treats of Bridges, on Trestles, Piles, Truss-frames, and other applications of Carpentry. This section is managed with the same ability as the other parts of the volume, and cannot fail to be highly useful and instructive to military engineers, though, from the length to which this article has already extended, we are reluctantly forced to dispense with an analysis of its contents. To this is subjoined a valuable appendix, on the strength of timber and other materials, of great importance to all descriptions of practical men, of either the civil or military professions. We now terminate our examination of this most useful and interesting volume, with Segur's graphic description of the passage of the Beresina by the French army under Napoleon, in his disastrous retreat from Russia in 1812; and the author's judicious reflections on the propriety of keeping up, even in peace, a corps of pontoneers. "When the Imperial Guard began to move, the stragglers dispersed in the surrounding woods and villages, who had not taken advantage of the first night to cross the river, now rushed from all sides, and flocked to the river side in one dense and confused mass, which soon choked up the narrow entrances to the bridges. The foremost, impelled by those who followed, were driven upon the guards and pontoneers, who were endeavouring to keep a passage open for our troops. In repressing these crowds of fugitives, many were trodden under foot in the mêlée, others thrown upon the floating ice, and great numbers, unable to regain the bridge or to reach the shore, perished in the river. The efforts of Napoleon and his officers to re-establish order were unavailing. So great was the confusion, that force was necessary to clear a passage even for the Emperor, who crossed at about 2 P. M., with nearly 6000 of the guard under Ney. The troops continued to pass till the morning of the 29th; but multitudes of the stragglers, benumbed with cold, were unable to avail themselves of this last opportunity; and about 8 in the morning, General Eblé, seeing the Russians advancing, was obliged to set fire to the bridges, leaving vast quantities of ammunition, artillery, and baggage, thousands of men, and many women and children, to the mercy of the enemy, and to the rigours of a merciless climate. 66 Napoleon is said to have exclaimed, when he heard that Tchaplitz (the Russian General) had abandoned his position- Ah! J'ai trompé l'Amiral ;' and certain it is, that if Tchitchakof had not descended the Beresina, directing Tchaplitz to move in that direction likewise, upon the supposition that Napoleon would not attempt the passage above, his further retreat would have been intercepted there. The escape of Napoleon was thus so much owing to the foresight, ability, and enterprise of General Eblé, and to the intelligence and experience of the corps of pontoneers, by which Napoleon was enabled to profit promptly by this mistake, that these details cannot be too forcibly cited, as proof of the vast importance of our keeping up, during peace, the nucleus at least of a corps, which cannot be formed upon an emergency. The multifarious matter contained in this book all appertains to its duties, and may show, that if, by inconsiderate economy, we reduce the establishments in which only these executive qualifications can be perfected, we shall unquestionably, some time or other, have again to encounter difficulties, such as those which were experienced in all the early operations of the late war. It is a sage maxim in state economy, that, in proportion as constitutional, political, or financial, circumstances render it expedient to reduce the numerical force of its army during peace, it becomes the more necessary to maintain the establishments in which practical science is upheld; and no one, I think, who reflects upon the very extensive, important, and elaborate attributes of the corps upon which such services as these depend, can doubt the expediency of preserving such a nucleus of it as may insure the retention at little cost, of what has been acquired in a long series of arduous and expensive training on actual service." To these striking and forcible remarks, which carry conviction to every mind capable of just reflection, little need be added by us to recommend the protection of our seminaries to public notice. If we allow our scientific institutions, either naval or military, to be injured or suppressed, we may venture to predict that the country will pay dearly for the sacrifice. Euler and Bouguer were honoured and rewarded for their theoretical investigations of the principles which should be observed in shipbuilding and naval equipments, and their efforts have been long apparent in the superior qualities of the vessels in the French marine. Before the establishment of our naval and military colleges, their engineers were equally superior to ours, though they were perhaps inferior towards the close of the late war. It remains with the government of the country to determine what must be our future status as engineers in either service. Experiments on the Expansion and Contraction of Building Stones, by variations of Temperature. By WILliam H. C. BARTLETT, Lieutenant United States Engineers *. SIR, Fort-Adams, Newport Harbour, March 12. 1832. IN the progress of this work, we have had occasion to use considerable quantities of coping-stones taken from different localities, with all of which it has been found impossible to obtain tight joints. The walls on which these stones were placed have not undergone the slightest change; and notwithstanding they were laid with the greatest possible care, and their joints were filled with the best cements that could be devised, yet, at the expiration of a few weeks, these joints were broken up by fissures which extended from the top to the bottom of the coping. These fissures were supposed to have arisen from a change of dimensions in the coping-stones, in consequence of the ordinary variations of atmospheric temperature; and, with the view to ascertain if the total amount of cracking could be attributed to this cause alone, a series of experiments was instituted by order of Colonel Totten, and continued from 18th August 1830 to 2d June 1831. The circumstances connected with these experiments, as well as their results, you will find subjoined. Colonel Totten requests me to communicate them to you, supposing that you may find them of sufficient practical importance to deserve a place in your Journal. These experiments were made nearly at the same time, upon granite, limestone, and sandstone, the kinds of stone used for the coping; and for this purpose a piece of each was selected, in such a manner that the three pieces were of nearly equal lengths. The granite has a fine grain, is of a compact texture, and was taken from a boulder at the head of Buzzard's Bay : the limestone is white, has a fine grained crystalline structure, and accompanies primitive rocks; it was taken from the quarries of the Sing-sing State Prison, New York: the sandstone is from the quarries in Chatham, Connecticut, and belongs to the old red sandstone formation, according to the Rev. Edward Hitchcock;+ * Silliman's Journal, Vol. xxii. No. 1. April 1832. † He now refers all the sandstone of the Connecticut valley to the new red sandstone. it has a granular structure, rather coarse, and its cement is argillo-ferruginous. To ascertain the exact lengths of these pieces at the different temperatures produced by exposure to the weather, these alone being important for our immediate object, and for the purposes of construction generally, the measurements were made by means of a white fine rod, with copper elbows at the ends, embracing the stones when applied to them, as represented in the sketch. gyeri hotelą base angle AA is an elevation or vertical section lengthwise of the stone to be measured; BB the measuring rod, with elbows D and C, of thin hammered copper, firmly secured to it. The end D was always adjusted to the same part of the stone, by sliding through a groove in the copper guide F cemented to the stone; the elbow C was adjusted in like manner by sliding through a groove in the piece E, also attached to the stone. The elbow C has itself a groove, through which the wedge W may slide horizontally under the guide E, between the elbow C and the stone. This wedge being graduated as a diagonal scale, showed by the distance which it entered, the difference between the length of the measuring rod and that of the stone. The expansion of the measuring rod being known, the length of the stone could be calculated in decimals, viz. the English standard inch. A groove was cut in the stone, in which a thermometer was placed at each measurement, and, being covered, was suffered to lie some time, in order to ascertain the temperature of the stone. The temperature of the measuring rod was assumed to be that of the open air to which it had been exposed. By Lardner and Kater's Mechanics, we have, as a mean between the results of Captain Kater and Dr Struve, for the linear expansion of deal wood, in terms of its length, for one degree of Fahrenheit, the decimal .00000255; and by the EdinVOL. XIII. NO. XXVI.-OCTOBER 1832. U |