The Spanish gunboats, thirty in number, recently seized by the United States Government, are perhaps an instance of t-e most rapid naval construction on record. As anything pertaining to these vessels is now a matter of current interest, and will be of historical importance in the future, we have visited the Delamater Iron Works, at the foot of Thirteenth street, North River, this city, where the fleet is at present lying, and have gained the following particulars. The contract was entered into May 5th, and the first keel was not la, until the 19th of the month ; yet on the 23d of June the first vessel was launched, and by the 3rd of September the last of the thirty was floated. They are all built after one design, prepared by Capt. Ericsson, of monitor fame. They are twin screw steamers, 105 feet in length, 22 feet beam, and their depth of hold is 8 foot. Their maximum draft is, we are informed, to be 5 fet. The screws are 5 feet 10 inches in diameter, and are each driven by two steam cylinders, the length of stroke being 14 inches, and the diameter of the pistons 15 inches. The arrangement of the steam machinery is the most compact we have ever seen, the whole, including pumps, surface condenser, fresh water generator, etc., resting on a surface of ten feet square and rising from this surface only six feet. The nOOlinal horse-power of the engines in each boat is 140. They were set up in the works, and placed bodily on board the gunboats in the following manner: A model of the bed plate of the machinery was made of wood, this was taken into the vessel through the boiler hatch, and lined properly, when it was taken out, and the engines were in turn let down, hauled aft by steam, and fastened down. So expeditious was this process, that on a single afternoon, between one and six o'clock, the machinery was placed in three boats. There are other peculiarities of the engines which it might be interesting to notice, but we must pass to other features of these in some respects unique war vessels. The lines are made full at the bow, in order to support a heavy bow-gun. This gun is to be an improved 100-pound parrott rifle, mounted upon Capt. Ericsson's new gun carriage. This carriage and the arrangement of the engines above described, are novel and striking features oi these boots. The rigging is of wire, and the masts and smoke-pipe are given more rake than usual. A peculiarly light and graceful appe'arance is thus imparted. A preliminary trial of the gunboat first finished was made in September, and the official trial took place Oct. 25 th. The results of both these trials were very satisfactory.. The ground selected for the official trial was from Fourteenth street to One Hundred and Twenty-Ninth street, on the Hudson River, a distance of 5'81 statute miles. The run up the river against the tide occupied 32 minutes and 35 seconds, and the return trip was made in 29 minutes and 35 seconds. Total distance, 11-62 miles. Total time, 62 minutes, 10 seconds. Considering the small size and full lines of these gunboats, the speed attained is considered remarkable. The vessel was loaded with pig iron during the trial, to her intended maximum draft. The execution of so large a contract in so brief a time by a WIgle establishment, is a marvel of rapid work, but when it is remembered that the inside finish is much more elaborate than that usually given to boats of this class, and that the contract included the entire outfit of the fleet, it must be considered as almost an unparalleled achievement in its way.