vessels, any three of which being stationary, the distances and posi tions of objects could be determined from them by direct angles, giving a double result, or by the angles between them, taken from the shore or from boats, furnishing data for the problem of "the three points." When both methods can be applied at the same time, it is evident that the utmost accuracy may be obtained. Upon the land, by employing many observers, and occupying all the points of a trigonometric survey simultaneously, the whole work might evidently be completed in a very short space of time; and in like manner upon the water, creating, by means of vessels and boats, a number of artificial stations around an island, measuring angles simultaneously at them all, and measuring the base lines by sound, the rapidity with which a survey can be performed is equally as great. By means of the system of signals prepared by me for the Exploring Expedition, I could direct the vessels to assume any position I might select as most fit for our purpose. When these were reached, general but minute instructions, directed the observations that were to be taken at each, in doing which there was no difficulty. These observations were entered upon a deck-board, for which the following form was prescribed: To illustrate still further the mode in which the whole squadron was made to concur in obtaining unity of action, a detail of a survey and a plot of the work are subjoined. On approaching the island to be surveyed, signal was made to prepare for surveying duty; if boats were to be used, the number of them and the vessels whence they were to be despatched was next indicated; and finally, the position each vessel was to occupy was shown. The vessels having reached their assigned places, hove-to and the boats, having been previously despatched, would about the same time have anchored in their assigned stations, at the points of reefs, and hoisted their appropriate signals. The vessel that is first to fire a gun then hoists an ensign at the foremast-head, which is answered by all, and the flag is in like manner displayed previous to each suc cessive fire. The firing then goes on in quick succession from all the vessels; and at the time, all the officers being on deck for the purpose, angles are measured between the other vessels and objects on the shore, each by a different observer, as directed and indicated by the oldest officer on the deck; these angles, together with those made by the visual tangents to the shores, have always been found sufficient to plot from. During this operation the ensign is kept hoisted at the peak, so that all the angles may be taken simultaneously. Altitudes of the sun for time, and angles whence to calculate the azimuth of some one of the objects, are also taken immediately before or after the horizontal angles; and at the same time, the altitude and azimuth of the mast of one of the ships is observed for the purpose of defining their position. When this is done, one of the vessels changes her position, while the others maintain theirs by manoeuvring so as to keep the angles between objects on the shore of the same magnitude; the boats having in the mean time changed theirs also: the same operations are then repeated. Of such an operation, the annexed wood-cut is an example. By these alternate changes in the stations of the several vessels, and boats continued until a circuit of the island has been made, the work is finished, and when it joins, it is proved by the last distance determined by azimuthal angles and sound, as by a base of verification. The deck-boards are now sent on board the flag-ship, where the work is calculated and plotted. The survey of the island thus represented, which is about seven miles in length, was performed in three hours and thirty-five minutes. It began, as noted on the deck-boards, at 1h 18m P. M., and the observations closed at 4h 53m P. M. INSTRUCTIONS RELATIVE TO THE DECK-BOARD. THE deck-board will be kept strictly according to the formula herewith sent. The officers of the deck will be particular in marking the times at which any of the vessels change their position, and the length of time they remain in them (by ship's time); also the times that the firing takes place. The measured angles will be at once communicated to this ship. The original deck-paper, with the calculated azimuth results, will be sent to me at the expiration of the day's work, a copy of which will be made previously, and kept on board. The particular attention of the officers is required to the "Instructions for Boat Duty, Surveying, &c.," of the 22d of February, 1839, also to the Manual of Surveying. CHARLES WILKES, Commanding Exploring Expedition. U. S. Ship Vincennes, At Sea, August 15th, 1839. XLII. U. S. Ship Vincennes, At Sea, August 25th, 1839. SIR, I was surprised to notice this evening, that the boats from the Peacock were not alongside at sunset, as there are positive written orders to that effect, unless the boats are previously recalled by signal. I presume, also, that the officers of the boats had no authority from you to land, although I saw several on shore, which must have caused some delay in regard to those engaged on surveying duty, a service second to none in the Expedition; besides hazarding a difficulty with the natives, which might have occasioned also a delay to the whole squadron, as they are evidently hostile. I trust a similar occurrence will not come under my notice again during the present cruise. The orders to the officers of the boats must be well understood before they leave the ship, and strictly obeyed; otherwise the intercourse with the islands must be restricted. HEREAFTER no specimens of coral, live shells, or any thing else that may produce a bad smell, will be taken below the spar-deck, or into any of the rooms; and it will be the duty of those bringing such on board, to report them to the officer of the deck, who will see that they are placed under the charge of a man appointed to look out for them; and that this order is strictly obeyed. CHARLES WILKES, Commanding Exploring Expedition. XLIV. U. S. Ship Vincennes, At Sea, September 1st, 1839. SIR, You will proceed around the island of Raraka, and obtain its dimensions, as indicated in your orders of yesterday, turning its west point, and continuing down its south side, until you reach the position you left yesterday. Your courses and distances must be kept very accurately, and your approach to the shore and reef, as near as practicable with safety. You will always regain the same position in the morning, that you left the day before. After you shall have executed this duty, you will then proceed to the northeast, and make the island we discovered, called Taiara, or King's Island, and steer from there by daylight a northwest course. You will make Carlshoff Island, and from thence proceed to the north side of Prince of Wales Island, where you will meet me; if not, however, you will determine the east and west ends of it, and that of Krusenstern's Island in its vicinity; from thence you will proceed to Matavai Bay, and await further orders. You will proceed to windward, and visit Oura and Tiokea Islands, of King George's Group, and ascertain their correct position, and the number of islands composing the group. From thence you will proceed by Waterlandt to the Prince of Wales Island, coasting along the northern side, and taking observations as required by my orders of the 1st instant; and from thence to Matavai Bay for further orders. Should you arrive at Matavai Bay before this ship, you will allow none of the natives to remain on board or visit the Flying-Fish after sunset. Your vessel must never be left without two officers. LIEUT.COM. R. F. PINKNEY, CHARLES WILKES, Commanding Exploring Expedition. END OF THE FIRST VOLUME. |