The screw frigate Arethusa, 35, 500-horse power (nominal), Capt. Reginald J.J.G. Macdonald, one of the vessels composing the experimental steam squadron, having completed the repairs and alterations to her machinery rendered necessary from the discovery of several serious defects on the occasion of her former trial, was yesterday taken to the Maplin Sands for the purpose of testing her final rate of speed, previously to her departure to join the Octavius and the Constance, the two other vessels which will form the squadron. The Arethusa was originally constructed as a sailing frigate from the designs of Sir W. Symonds, but was subsequently cut asunder, lengthened, and converted into a steam frigate at Chatham dockyard, and was the last of the vessels so treated at that establishment. She is 252ft. in length, with a breadth of beam of 52ft. 8in., while her displacement is rather more than 3,500 tons. She is fitted with a pair of horizontal trunk engines, with surface condensers and other improvements, her machinery having been manufactured by Messrs. John Penn and Sons. The Arethusa, unlike the other vessels of the squadron has only two cylinders, while the engines of the Constance, fitted by Messrs. Randolph and Co., has six cylinders, and the Octavia, the machinery of which was manufactured by Messrs. Maudslay and Co., has three cylinders, with their cranks placed an an angle of 120 deg. with each other, by which, it is contended, the power exerted is rendered more uniform. The object of the trials ordered to be undertaken by the three vessels of the squadron is to enable the Admiralty to ascertain the best form of marine engine to be in future adopted in the service, particular regard being had to the comparative saving of fuel by using superheaters, surface condensers, and the other improvements effected in the machinery of each of the vessels. The cylinders of the Arethusa have a diameter each of 79.96 inches, and a length of stroke of 3ft. 6in. The screw is an improved two-bladed Griffiths propeller, with a diameter of 18ft., and is set at a pitch of 21ft. 6in., the centre being 12ft. 2in. above the bottom of the false keel, with an immersion of 2ft. 2in., the screw’s pitch to the diameter being 1.19. On leaving her anchorage yesterday the Arethusa drew 23ft. 4in. aft, and 20ft. 8in. forward, the total quantity of coals on board being about 200 tons, with 240 tons of shot, shell, guns, and War Department stores. The day was exceedingly favourable for the trial, the sea being perfectly calm, with no wind. The vessel was in charge of Commander Barrow of the Steam Reserve, the other officials on board being Mr. Tucker, principal inspector of machinery afloat; Mr. Blaxland, inspector of machinery; and the officers connected with the master-shipwright’s department. The Nore-light was reached at 11 a.m., and the frigate’s head at once laid for the measured mile, where the customary number of runs was made, but owing to some cause the feed pipes in the boilers became choked just as the frigate was about entering on her fifth mile; while at the same time the boilers commenced priming to a considerable extent. The following were the results of each run: First mile – time, 6min. 46sec. ; speed in knots 8.867 ; pressure of steam 17lb. ; vacuum, 22 ; number of revolutions, 65. Second mile – time, 4min. 42sec. ; speed 12.776 knots; steam, 19lb. ; vacuum, 22; number of revolutions, 66. Second mile – time, 4min. 42 sec. ; speed, 12.776 knots; steam, 19lb. ; vacuum, 22; number of revolutions, 66. Third mile – time, 6min. 53sec. ; speed, 8.716 knots; pressure of steam, 15lb. ; vacuum, 22.5 ; number of revolutions, 64. Fourth mile – time, 4min. 52sec. ; speed, 12.329 knots ; pressure of steam, 19lb. ; vacuum, 22.5 ; number of revolutions, 64. At the termination of the fourth run, from the causes already stated, a delay of between two and three hours occurred, and the remaining two miles were afterwards ran with the following results :– Fifth mile – time, 5min. 23sec. ; speed, 11.145 knots ; pressure of steam, 16lb. ; vacuum, 24 ; number of revolutions, 61. Sixth mile – time, 7min. 11sec. ; speed, 8.352 knots; steam, 19lb. ; vacuum, 22.75 ; revolutions, 63. Taking the mean of the above figures the true average speed of the vessel was found to be 10.82 knots per hour, showing a falling off of nearly a quarter of a knot per hour, which is attributed to the foul state of the ship’s bottom from the length of time she has lain in harbour. Two runs were afterwards taken at half-boiler power, when the mean speed realised was 8.117 knots, with 17lb of steam and 50 revolutions of the screws. At the close of the trials, which were prolonged until nearly 7 o’clock last evening, the frigate’s head was laid for the Little Nore, where on bringing up she saluted the flag of Vice-Admiral Sir C. Talbot, K.C.B. Immediately afterwards considerable excitement was occasioned by the report that the vessel was on fire, which was confirmed by the fire-bell ringing and smoke being seen pouring up from the fore gratings. Immediately the alarm was raised the crew were at their stations, and the pumps got ready and at work, a signal being in the meantime made to the Formidable flagship of “Ship on fire!” The fire was supposed to be in the neighbourhood of the fore boilers, to get at which a portion of the lower deck was cut away, and an abundance of water poured down, the pumps, on the whole working satisfactorily. As soon as the signal was made, boats from the various vessels in harbour, all filled with seamen, were despatched to render assistance. About an hour after the fire was reported it was so far got under that all cause for apprehension was at an end, although its exact locality even then could not be accurately known. The Arethusa was to have left for Portland at 3 a.m. this morning, to join the other vessels of the squadron, but in consequence of the occurrence of the fire she will be delayed until a report of the whole of the circumstances has been made to the Admiralty, and the vessel has undergone a thorough overhaul at the hands of the dockyard officials.