The work of constructing the bridge will occupy, say, a year and a half.

PLAN NO. 2.

High bridge. Superstructure of main bridge.

There will be over the river six spans of 420 feet each, and one span of 130 feet, measured between centers of masonry, making the entire length of the main bridge, 2,650 feet. The bridge will be for two railroad tracks, and the superstructure will be, say, 50 feet wide over all. The trusses will be, say, 50 feet high. From ordinary surface of water to the extreme under-side of the superstructure will be, say, 150 feet. Superstructure will be of iron throughout.

Substructure of main bridge.

Substructure to consist of two land-piers and six channel-piers. Foundations of piers to be the same as described for the piers in plan No. 1, except that the timber grillage will be, say, 8 feet thick. Piers will be of masonry reaching from the top of the timber grillage, say, 10 feet below the water, to a point 20 feet above water, making a mass of masonry 30 feet high. Masonry at top will be 23 feet wide and 82 feet long, battering as usual downwards on all sides, except at the upperstream end, which will be provided with a suitable ice-breaker; size of pier at bottom of masonry, say, 28 feet wide and 98 feet long.

On the top of the masonry will be placed a wrought-iron trestle, consisting of ten vertical and four inclined columns, properly braced and tied together; at the top of this trestle will be, say, 6 feet wide, and 52 feet long between centers of exterior columns; and at bottom 17 feet wide and 74 feet long, its height will be, say, 132 feet, making the verti cal distance from the surface of the water to the top of the trestle, say, 152 feet.

Approaches.

The main bridge will be reached from each side by a curved approach. constructed with a gradient-rising toward the bridge at the rate, say, of 100 feet per mile. The total length of each of these approaches will be, say, 6,600 feet, of which 1,200 feet at the lower end will be earth embankment, and 5,400 will be iron trestle-work, arranged in spans of, say, 30 feet each. The iron-trestles will vary from 25 to 150 feet in height.

Cost of the bridge.

The estimated cost of the bridge, if built on this plan, is as follows:

The work of constructing the bridge will occupy, say, three years.

PLAN No. 3.

Winter bridges. — Superstructure.

There will be one pivot draw-span 250 feet long, leaving, when open, a passage way 100 feet wide, in clear, on each side of the pivot pier; eight permanent fixed spans of 200 feet each, and two moveable spans 400 feet each, making a total length of 2,650 feet. The bridge will be for two railroad tracks, and the superstructure will be, say, 32 feet over all. The trusses will be, say, 28 feet high. From ordinary surface of water to the extreme under side of the superstructure will be, say, 12 feet. Superstructure to be of iron throughout except the temporary moveable spans crossing the 400 feet opening, which will be of wood. Each opening of 400 feet will be crossed by two spans of 200 feet each. These spans will be supported where they meet in the center of the opening by a pontoon. Just above the pontoon will be placed a heavy floating timber crib of such construction and strength, and so securely anchored as to afford the pontoon protection from the ice. The temporary spans, the pontoons, and the floating cribs to be removed from the channel during the season of navigation.

Substructure.

The size, shape, and construction of piers and foundations to be of the same general kind described above, for plan No. 1.

Cost of bridge.

The estimated cost of the bridge, if built on this plan, is as follows:

The work of constructing the bridge will occupy, say, one and a half years. WILLARD S. POPE, Civil Engineer.

DETROIT, August, 1873.

IX.

To the Board of Engineers:

Please allow me to present the following considerations regarding the respective merits of a high bridge and a low bridge over the river at Detroit:

It is legitimate to assume that capital invested should receive a fair return in the shape of interest. The only way in which such interest can be derived for capital invested in transportation facilities is by taxing the goods transported. Assuming the investment to be a judici ous one, this tax must always be large enough to cover the fair interes thereon, as well as the current expense of doing the business and of making good the usual and necessary wear and tear. The larger the investment the greater the tax. And this tax falls of course ultimately npon the consumers of the goods transported-that is to say, upon the public. Therefore, any policy or system which increases beyond what is absolutely essential either the first cost of a railroad or its necessary operating expenses, or both, becomes at once a permanent public bur den, while any policy or system which diminishes such cost is a public benefit.

Of the plans proposed for the Detroit bridge, two are prominent, viz, one for a low bridge, with two draws, estimated to cost $2,500,000, and the other for a high bridge, estimated to cost say $9,000,000, a difference of $6,500,000, the annual interest on which, at 7 per cent., is, say, $455,000. The former is on a level with the railroads now or hereafter to be built, while, to reach the latter, all trains must forever climb 150 feet. The extra cost of making this ascent may be assumed at, say, $50,000 annually. Taking the annual expense of repairs and renewal of the trestle approaches to the high bridge at 5 per cent. on their first cost, ($3,100,000,) this item will amount to, say, $155,000. The annual extra cost will therefore be :

Interest on first cost

Extra cost of working .

Extra cost of repairs.

Which, capitalized at 7 per cent., is $9,430,000.

$455,000

50.000

155,000

660,000

This amount may be fairly assumed therefore as the real difference in cost between the high and the low bridge, so far as the items above are concerned, and the interest on this amount ($660,000) the public must annually pay for the luxury of the high bridge over what the low bridge would cost them.

Now, the amount of damage, whatever it may be, to navigation interests, from the presence of the bridge, forms a legitimate charge upon the goods transported by water, and must be paid by the consumers thereof, that is, by the public. Assuming the high bridge to inflict the minimum damage, it may be said that it is of public interest that this

form be adopted. But its adoption, as shown above, entails a permaBent and inevitable expense to the public (through the railroad charges) of $660,000 annually. If a low bridge be adopted, this can be forever saved; against which saving it is fair to offset simply the increased amount of damage to navigation interests from a low bridge over that from a high bridge. If this increased damage should amount to just $660,000 per year, the two forms of bridge would be exactly on a par, so far as the public interest is concerned. If it should exceed that amount, the high bridge would be an economy. If it should fall short of that amount, the low bridge will be an economy.

I have no estimate of the extra damage that navigation interests would sustain from a low bridge over that from a high bridge at Detroit, but to suppose that it would amount annually to $660,000, or the half or the quarter or the tenth of that sum, seems to me absurd. If this reasoning is correct, it is respectfully submitted that every consid eration of public economy dictates the adoption of a low bridge.

In addition to this, the elevation of the bridge (150 feet) is equivalent to removing Detroit one and a half miles away from the railway. All the local business of the city as connected with the railroad must be transacted at the foot of the gradient of the approach. Detroit will be located no longer on the main line, but on a branch or spur track; and all the evils will be experienced in perpetuity which are set forth in that part of Mr. Joy's communication to your board which refers to the location of the bridge at a point near Fort Wayne.

Very respectfully,

DETROIT, November 17, 1873.

WILLARD S. POPE.

X.

To the board of commissioners to inquire into the practicability of bridging Detroit River :

For the purpose of harmonizing, as far as possible, the apparently conflicting interests between the vessel-owners and the railroad companies, we herewith submit the proposition on behalf of the Michigan Central and Canada Southern Railroads, that a grant from the Government, allowing said roads to construct winter-bridges over the Detroit River, with two openings of 400 feet each, and a draw of 166 feet to each bridge, will be cheerfully accepted and acted upon by said rail

roads.

A. B. MAYNARD,

Of Counsel for Michigan Central Railroad.
E. W. MIDDAUGH,

Of Counsel for Canada Southern Railroad.

Dated DETROIT, November 18, 1873.

XI.

DETROIT RIVER TUNNEL-A PAPER BY ENGINEER CHESBROUGH.

The following paper on the Detroit River tunnel was read by Mr. E. S. Chesbrough, its engineer, before the Society of Civil Engineers at Louisville :

At the date of the former paper on this subject, read at the last convention, the preliminary work on the Detroit River tunnel was in a very encouraging state. The Detroit shore-shaft had been sunk, and a drainage-tunnel extended from it for about 600 feet toward the Canada end. The Windsor shore-shaft had been sunk to below the bottom of the drainage-tunnel, which had progressed 100 feet toward the Detroit end. With the exception of finding harder ground, and consequently making slower progress than had been originally effected, the prospect of a successful completion of the work was brighter than at its inception, since previous to sinking the Detroit shaft there was a fear that very troublesome veins of water, supplied from the land, and having a higher source than the river, might be met. For this reason the Detroit shaft was sunk first, as the borings on the Windsor side did not indicate such veins of water.

In the latter part of July, 1872, when the work on the Windsor end had progressed about 250 feet through, for the most part, very hard ground, some of which was blasted, a sudden irruption of sand and water occurred which threatened to fill the tunnel out to the sump and choke the pumps. To prevent this a bulk-head was constructed near the face, but before it could be made sufficiently tight the workmen had to retreat some distance to make an apparently successful stand; and even this did not prove sufficient, so that a third and last bulk-head, still nearer the shaft, was put in. This state of things looked very discouraging, and it was, of course, impossible to tell the exact nature and extent of the source of the irruption, or how long it would continue.

From the character of the water itself, as well as from other circumstances, it evidently did not come from the river, and there was reason to hope its flow would soon diminish. This hope was not disappointed, and about the 14th of August the face was again reached, the bulkheads having been removed. Regular operations were resumed, but, after 30 feet of new tunnel had been built, a fresh irruption of sand and water occurred, making it again necessary to put in bulk-heads, preventing further advance for four days more. By this time it was concluded that the source of the irruption must be a vein, and not merely a pocket of sand; still it was hoped that it might prove quite limited in extent, and soon be passed. On the 12th of September, after the work had been extended 47 feet further, a third irruption occurred. After another placing and removing of bulk-heads, and taking out of sand, causing a delay of five days, regular operations were resumed, and 10 feet advance made when a further irruption occurred.

By this time the contractors had become very much discouraged, and felt that to continue the drift on the same level would be ruinous to them, as the work was costing more than four times the price they received for it. Inasmuch as the work on the Detroit side had been extended about 1,200 feet, sufficiently far to drain the lowest portion of the main tunnel, and inasmuch as the principal object now remaining was to explore the ground through which the main tunnel was to be built, it was decided to make a "lift-shaft" at the end of the drift, on