In the United States courts it is commenced by the filling of a libel and the arrest of the vessel to the mismanagement or fault of which the injury is Imputed. In In the English admiralty the suit is com- menced by the arrest of the vessel and the filing of a petition. In England, the judge is usually assisted at the hearing of the cause by two of the Masters or Elder Brethren of Trinity House, or other ex- perienced shipmasters, whose opinions upon all questions of professional skill involved in the issue are usually adopted by the court; 1 W. Rob. 471; 2 id. 225; 2 Chit. Genl. Pr. 514. In the American courts of admiralty, the judge usually decides without the aid or advice of experi- enced shipmasters acting as assessors or advisers of the court; but the evidence of such shipmasters, as experts, is sometimes received in reference to questions of professional skill or nautical usage. Such evidence is not, however, admissible to estab- lish a usage in direct violation of those general rules of navigation which have been sanctioned and established by repeated decisions; 2 Curt. C. C. 141, 363. When a party sets up circumstances as the basis of exceptions to the general rules of navigation, he is held to strict proof; 1 W. Rob. 157, 182, 478; 6 Thornt. 607; 5 id. 170; 3 Hagg. Adm. 321; and courts of ad- miralty lean against such exceptions; 11 Ν. Y. Leg. Obs. 353, 355. The admissions of a master of one of the colliding vessels sub- sequently to the collision are admissible in evidence; 5 E. L. & Eq. 556; and the masters and crew are admissible as wit- nesses; 2 Dods. 83; 2 Hagg. Adm. 145;8 id. 321, 325; 1 Conkl. 384. As to the burden of proof in collision cases, see 9 Jur. 282, 670; 2 W. Rob. 30, 244, 504; 12 id. 131, 371; 2 Hagg. Adm. 356; 4 Thornt. Adm. 161, 356; 1 Conkl. 382; 1 How. 28; 18 id. 570; Olc. 132; 55 Fed. Rep. 338; [1892] Prob. 419. The general rules in regard to costs in col- lision cases, in the admiralty courts, are that if only one party is to blame, he pays the costs of both; if neither is to blame, and the party prosecuting had apparent cause for proceeding, each party pays his own costs, but in the absence of apparent or prob- able cause the libel will be dismissed with costs; if both parties are to blame, the costs of both are equally divided, or, more gener- ally, each party is left to pay his own costs. But costs in admiralty are always in the discretion of the court, and will be given or withheld in particular cases without re- gard to these general rules, if the equity of the case requires a departure from them; 2 W. Rob. 213, 244; 5 Jur. 1067; 2 Conkl. 438. Consult 2 Pars. Mar. Law, 187; Conkl. Adm. 370-426; Fland. Mar. Law, c. 9; Abb. Shipp. Story & Perkins's notes; Marsden; Preble Spence, Collisions.