(after stating the facts as above). That portion of the abutment below the surface of the water was, by reason of its position and peculiar construction, a concealed and imminently dangerous obstruction to the navigation of the river. As it was placed there by the city of Chicago, so it became tiie duty of the city, so long as the obstruction was maintained, to so guard it that injury therefrom should not result to vessels navigating the river. Philadelphia R. Co. v. Philadelphia, etc., Towboat Co., 23 How. 209. That duty was sought to be discharged by maintaining spiles along the face of the obstruction, which were effectual to prevent vessels passing the draw of the bridge from coming in collision with the projecting footing courses of the abutment. The maintaining in place of these spiles, in the absence of other safeguards, was an imperative duty. Their removal was an act of negligence contributing to produce the injury here complained of. Acting under authority of the city, the contractor, as well by its contract as by the law, was liable for damages arising from the removal of the protection. The duty is the same whether an obstruction to navigation be created or a protected obstruction be uncovered. The contractor undertook to perform the work of removing the *629stonework, and to “keep free and unobstructed the channel of the river, so as not to interfere with or obstruct tbe movement of vessels or other craft,” and to “maintain suitable signals and lights to be approved by the commissioner of public works as a warning to vessel men.” This, however, merely emphasized an imperative duty imposed by law'. The contractor is not absolved by Ms ignorance of the character of the construction below the surface of the water-. Undertaking the work, he is chargeable with knowledge of tbe character of the work he undertook to perform. Removing protection work, whose obvious function was to protect vessels from contact with the abutment, the contractor is chargeable with knowledge of the fact of the obstruction and of its nature. Tin; law cannot permit one to uncover a, concealed and dangerous obstruction to navigation, and to plead in excuse ignorance of the character of the obstruction,. He must act at bis peril. Acting under tbe authority of the city that created the obstruction, and bound by the contract to protect the city, and bound by law to keep free and unobstructed the channel of the river, if, in the prosecution of the work, it became necessary to remove protection works that had been maintained by the city, and that guarded tbe navigation , of the river, the contractor could not, shut his eyes to the character of the obstruction that had so been covered, or negligently remain ignorant of its character. Tin; protection work itself was notification of a danger that: it guarded. Removing the protection, tin; actor was bound to know the character of the danger that he uncovered. The contractor knew of the submerged abutment, knew that it was dangerous, because protection work bad been maintained to guard against that: danger. Removing the protection and exposing the obstruction, the contractor is chargeable with knowledge of its character, and with the result, of his negligent act, in failing to substitute other and effectual safeguards and warnings. Casement v. Brown, 148 U. S. 615, 623, 13 Sup. Ct. 672. As matter of fact, the contractor bad sufficient notice of the character of the submerged obstruction to put one upon inquiry as to its character. In a narrow draw', only 16 feet: wide, the protection spiles were placed (5 feet away from the face of the abutment above the water, whereas at the other bridges in the city the protecting spiles are placed but two feet: distant. Naturally then; was a motive for this, when it is matter of common knowledge that such protecting spiles are driven close f.o the base of the abutment. The fact was sufficient fo put tile contractor upon inquiry, and failure; to inquire, when inquiry would have; disclosed the fact, was negligence.

We do not conceive that the contractor is excused upon the plea that the work in the north draw of the bridge had been finished. While the work was then; progressing, the draw had been closed to navigation, and certain vessels had anchored for winter quarters at the east entrance of the draw. We think it was the duty of the contractor, under such circumstances, before proceeding with the work in the south draw, and before uncover*630ing so dangerous an obstruction, to see to it that the north draw was free to navigation. The contractor commenced the work and uncovered the obstruction in the south draw while the north draw was as effectually closed to navigation as though it had been blocked with the derricks and pile-drivers of the contractor.

Nor do we think the contractor excused by reason of the alleged negligence of the tug. The injury resulted from the combined negligence of the contractor and the tug. Concurring negligence is not a defense, and does not relieve from responsibility, where a plain duty was owing, and there was neglect in its performance. In such case the admiralty apportions the damages between the tort feasors. It enforces contribution from both parties in fault to liquidate the injury done to a third party.

The Wallula cannot be charged with the negligence of the tug. The latter was not her agent, but an independent contractor, and wholly controlled her ■ movements. The Doris Eckhoff, 1 U. S. App. 129, 1 C. C. A. 494, 50 Fed. 134; The Niagara, 1 U. S. App. 658, 663, 3 C. C. A. 342, 52 Fed. 890; The T. J. Schuyler, 41 Fed. 477.

By the decree complained of the contractor is charged with a moiety of the damages only, unless the appellees should be unable to collect from the owners of the tug the one-half part of their damages aw'arded against the tug company. This is in accord with the settled principle of the admiralty, and is not. subject to criticism. The Alabama, 92 U. S. 697; The Atlas, 93 U. S. 302; The Juniata, Id. 340; The Sterling and The Equator, 106 U. S. 647, 1 Sup. Ct. 89; The Max Morris, 137 U. S. 1, 10, 11 Sup. Ct. 29.

With .respect to the claim of the contractor the Fitz Simons & Connell Company, in which the towúng company does not join, that the.Wallula is chargeable wútli gross negligence on the part of its officers subsequent to the collision, whereby the damage to the cargo was aggravated, we need only say that we have carefully examined the testimony, and do not think that the evidence bears out the contention of counsel. A review of the evidence convinces us that all proper efforts were taken in ascertainment of The injury and in protection of the cargo.

We think it equally clear that the tug was at fault. The Wallula was wholly under the control of the tug. and unable to help herself. The tug was “the dominant mind or will of the adventure” (The Fannie Tuthill, 12 Fed. 446), and took the whole responsibility of her navigation (The Express, 3 Cliff. 462, Fed. Cas. No. 4,209). The collision occurred in broad daylight, and under such circumstances the fact of the collision creates a presumption of negligence on the part of the tug. The Delaware, 20 Fed. 797. Engaging in the service of towing up and down the Chicago river, the tug was bound to know the channel, and whether, under the circumstances, it was safe to make the venture of passing the draw. The Margaret, 94 U. S. 494. And that obligation imposes upon the master of the tug, before undertaking the towúng, a knowledge of the condition of the bottom and of the depth of water in the river, and of the exist*631ence and location of any well-known obstruction. The Lady Pike, 21 Wall. 1; Pettie v. Towboat Co., 1 U. S. App. 57, 1 C. C. A. 314, 49 Fed. 464; The Robert H. Burnett, 30 Fed. 214. This is not a case coming' within the principle of the cases died by counsel. The Angelina Corning, 1 Ben. 109, Fed. Cas. No. 384; The Willie, 2 Fed. 95, 8 Fed. 768; The dames A.. Garfield, 21 Fed. 474; The Mary N. Hogan, 30 Fed. 927; The Pierrepont. 42 Fed. 687. In those cases the tugs were absolved of responsibility for collision of the tow with unknown obstructions. Here this sunken obstruction had existed for some 25 years, and had been guarded, to the knowledge of the master of the tug, by these protection spiles. Prior to undertaking this towage, he knew those spiles had been .removed. He knew those spiles stood out six feet from, and indicated that it would be dangerous to go nearer, the face of the abutment. He had seen these abutments in process of construction. The master of the tug sought to excuse himself by asserting that he supposed everything had been torn out when the piling was taken down, and yet the abutment stood there facing him with the protection gone. He confesses that his idea in towing (he Wallula, through the draw was ■ simply to “keep her away from ¡he visible thing'.” lie assumed that the abutment proceeded on the same angle to the bottom of the river, notwithstanding he knew that the spiles stood out six feet from the face of the abutment, while at other bridges they stood out; but two feet. With this knowledge, and this imperfect comprehension of his duty, and in view of the fact that the Van Schaick could give but little assistance in steering the Wallula by reason of the position of the Palmer in the channel, it was the duty of the captain of the Carpenter to have had the Wallula under control, and he should not have permitted her to enter the draw at such a speed and upon such an angle that collision with the abutment was inevitable. The decree will be affirmed.