FOR THE JOURNAL OF THE FRANKLIN INSTITUTE.

Description of an improvement on the Bridge patented by ITHEIL TOWNE, Esq., the term of which patent has expired. By ITHAMAR A. BEARD, Civil Engineer.

Elevation.

The Improvement consists in the Upright Plank, and the Iron Bolts.

The Bolts are marked with a full dot (.) The Treenails marked with an open dot (o)

Scale, twelve feet to an inch.

In 1836 I was requested to advise the best construction for a bridge to be erected over a branch of the Androscoggin river, at Brunswick, Maine, from the main land to an island.

Accordingly, I recommended Mr. Towne's bridge; and in making a model for the purpose, it occurred to me that a great addition might be made to the strength and durability of the bridge by the addition of a pair of upright planks (one on each side of the X work;) over one half the intersections, these planks to stand perpendicular like posts, and the treenails or bolts to pass through and connect the work and the uprights at each crossing.

My object and intentions were to prevent, in the greatest possible degree, the depression of the bridge, and, on trial, I found my expectations fully realized.

This bridge is 101 feet long, in a single span, and is built with a single travel only, twelve feet wide in the clear, and the travel of the bridge is arched, or crowned, sixteen inches in the centre above a level with the ends.

For the better convenience of drafting I have made the accompanying plans level from end to end, and in fact, in a bridge of this structure I cannot see that any benefit or advantage is derived by arching.

In this bridge, instead of the ordinary treenails, the principal intersections are firmly secured with bolts made of one and a quarter inch round iron, (Bank's best,) with cast iron washers, six inches in diameter, under the heads and nuts; a single bolt being used at each intersection.

There is a vast advantage in using the bolts, with heads and nuts, over the treenails, for, it not only holds the work together more firmly at first, but it likewise furnishes the means of keeping it always firm and close, for as the woodwork shrinks by seasoning or age, as is invariably the case, even if tolerably seasoned when worked, by turning up the nuts upon the screws occasionally, it may always be kept as compact as when first built.

Another advantage is, that the bolts need not be half so large as the treenails, and fewer in number are required, which very much saves the strength of the plank through which they are inserted.

The cost of building one of these bridges with bolts instead of treenails will be somewhat greater, but for a term of twenty or thirty years, I am fully of opinion that a bridge may be supported with less expense than if built wholly with treenails.

The bridge herein described was built by Capt. James Derby who now owns the Island and Mills thereon, and the work was most thoroughly put together in every part. The using of bolts instead of treenails was his suggestion.

After the bridge was completed, and before the trussels on which it was built were taken out, I set my levelling instrument and took the elevation of the travel of the bridge in the centre, and again after the trussels were removed, and it settled seven-tenths of an inch only.

This bridge has been in constant use ever since it was built, for teaming over with heavy loads of lumber from the mills on the Island, and it now stands as perfectly firm, true, and straight as it did when first completed.

To give a more clear idea of the cost of a bridge so constructed, I will insert the number of pieces and dimensions of all the materials of which this bridge is constructed, and also the cost for materials and labour as nearly as I can obtain them.

60 Do.

7 feet long, 6 by 4 inches.

840

Planking of the travel, 101 feet long 12 feet wide 3 inches thick 3636

Plank and Timber, in board measure,

28,006

Making allowance for waste, say 30,000 feet, worth here, at that time, $15 per thousand.

All the timber of this bridge is white pine.

The travel of this bridge is laid with the length of the plank lengthwise of the bridge. I do not think the plank will be so durable laid in this way as they would be if laid crosswise of the bridge, especially for heavy loads, and if planked crosswise a set of stringers must be laid on the crossills; say 5 stringers, 101 feet long 9 by 6 inches-2272 feet.

102 70

Taking the lumber of this bridge as it is S0,000, a $15 per thousand $450 00 632 cast iron washers, 3 pounds each, 2054 pounds a 5 cts. 304 Bolts 24 inches long?

34

66

12 66 Sweighing 2600 pounds. Estimation for heads, nuts and waste 400

66

Say 3000 a 10 c. including making 300 00 The labour was done by hands hired by the day, at an average of about $1 50 each per day, board included, and cost as nearly as can be estimated, about

Equal to twelve dollars a foot in length very nearly.

375 00

$1227 70

It is estimated that a bridge with two travels, fifteen feet wide each, with two sidewalks outside of the x work, and without a roof, may be built for twenty dollars a foot in length; timber and labour being at the same value as above rated.

To this must be added the cost of abutments and piers and of a roof if the bridge is to be covered.

There are no bridges in this section of country that stand so well and so durable as the "Town Bridge," and it is getting into general use. Brunswick, Maine, March 27, 1840.

Hydraulic Works at Algiers. By M. POIREL, Engineer of Bridges and

Roads.

[Translated from the Annales des Ponts et Chaussees by W. H. Emory, Jr., U. S. Topl. Engrs.]

The port of Algiers was established as far back as the year 1530, by Cheredin, brother of Barbarossa. Having made himself master of a little island, in front of the city, which the Spaniards had fortified, he resolved, in order to secure it, and at the same time to make, at Algiers, a harbour that would afford protection from the winds and from the swell of the sea, to unite it to the town by means of a jetty. This is called the Cheredin jetty, and is two hundred and twenty-three yards long and one hundred and twenty-seven yards wide. Its direction is nearly east north-east; or west southwest.

Besides the Cheredin jetty, another has been built on the prolongation of the island, which protects the harbour from easterly winds, and is called the mole. It is one hundred and seventy-four yards long, and forty-five yards in its greatest width. This mole runs north-east and south-west. These two

jetties with the little mole on which the Lazaretto stands form the boundary of the basin. It contains forty thousand seven hundred and twenty-two superficial yards, and can float sixty vessels, of which about thirty, may be vessels of three hundred tons, and some few, eight hundred tons. Vessels of a larger class anchor outside the basin. The greatest depth of water is sixteen and a half feet; but this may be increased by dredging. The Cheredin jetty and the mole were found in a state of complete dilapidation when Algiers fell into the hands of the French. These two works constructed of loose stone, (sometimes called rip-rap in the United States,) were levelled to their base. The Deys were in the habit every year of having the stones replaced which were carried away in the winter by the sea.

Laugier de Tassy, one of the most faithful historians of the Algerine regency, who resided there in 1727, says:―

"The great mole (the Cheredin jetty,) being entirely exposed to the north, to prevent it from being carried away by the furious swells of the sea, which roll up the sand bank, stretching along this mole and out into the sea, they were obliged to keep the slaves of the beylick employed the whole year carrying hard stones from a place near point Pescade, to put them along the mole. The sea soon carried away the stones thus deposited, but care was always taken that they should be replaced."

Large magazines of military supplies are placed on the Cheredin jetty and it naturally claimed the first attention of government.

The preservation of these magazines required that the loose stone upon which they rested, at the base of the jetty, should be secured.

This undertaking was confided to M. Noel, the engineer, in charge of the hydraulic works at Toulon, from which he was temporarily relieved.

He rebuilt the entire body of the jetty to a height of sixteen and a half feet above the water, with a thickness of six and a half feet. The new masonry is of the very best kind and possesses great solidity; unfortunately the insufficiency of funds placed at the disposal of the engineer and his limited time did not permit him to turn his attention to the foundation of the jetty which will soon require considerable repair.

The extremity of the mole, called the chop, in which the sea made a large breach, was repaired in 1831, but the new masonry being built upon the fragments which the action of the sea had brought down, was entirely destroyed by the first storm in the winter of 1832. All the repairs made to the top of the work were necessarily liable to the same catastrophe, as the base upon which they rested was insecure. It became necessary, therefore, before proceeding farther, to reconstruct the base permanently and durably.

The locality did not permit the engineer's resorting to the ordinary means of establishing a foundation by throwing over loose stones, (riprap.) The shore to the west, where the quarries are, has not a single creek or harbour where a vessel could load; it is open to the ocean and skirted by a reef of rocks which make the landing dangerous even in a calm. The transportation of blocks of stone could only be effected by carriages, a tedious and difficult operation with masses, which, to resist the action of the waves, should measure at least four cubic yards. Besides which it would have been necessary to carry these blocks through the most fre quented and populous part of the city, very much to the inconvenience of the inhabitants passing to and fro. Another difficulty presented inself, even if the obstacles to an easy transportation had been overcome. To give sufficient stability to the work at the end of the mole, a long slope of at least one in ten was necessary, which would have entirely obstructed the navigation, as the entrance to the basin was already very narrow, being

only one hundred and thirty-four yards wide, measuring from the end of the mole on which the Lazaretto stands, to that of the work in question.

Under these circumstances, the engineer was obliged to resort to other expedients, and he was thus led to form and execute a new plan for establishing foundations at sea, which five years experience of the works at Algiers has proved to be, according to all accounts, superior to all those which have heretofore been put in practice and particularly to those made of rip-rap work; a method much approved of since the construction of the Cherbourg and Plymouth breakwaters, the two most important maritime works executed in modern times.

The principle feature of this plan is the use of blocks made of béton. These blocks are of two kinds; one being constructed in the water at the place it is intended to occupy, the other made on shore and launched.

The first is made by pouring the béton into cases without bottoms, sunk on the place where the block is to rest. The frames of these cases as shown in the annexed cut, are made by putting together pieces of scantling, in a rectangular form, and the sides are made by nailing to this frame two courses of plank placed at right angles to each other. The lower edges of the cases are cut out to fit the profile of the surface on which they are to rest. They are lined with tarred cloth, throughout the whole extent of the inside up to the level of the water. The cloth at the bottom is allowed sufficient fullness to accomodate itself to the inequalities of the ground. The cases are thus, in fact, converted into cloth sacks, the sides of which are strengthened by the timber work on which they are stretched and fastened. The cloth sacks enable the mass of béton to accommodate itself perfectly to the surface which receives it, the inequalities of which serve to bind together the rock forming the bottom, and the béton. This is a great advantage in the use of these cases, for with the flat bottomed ones generally used, it is necessary to level the surface to be built upon, which is a difficult and uncertain operation.

The cloth bottomed cases are built upon stocks, launched and floated to the place they are to occupy. They are then sunk by means of small wooden boxes, one foot square, filled with cannon balls or pig-iron strung entirely around on the outside of the case, about one foot and a half from the top, by means of a cable passing through iron rings fixed in the uprights. The arrangements for sinking the cases is shown in the annexed figure.