triangulation. This plan involves less time and expense than those previously suggested, and dispenses with the use of reflecting instruments and of floating signals. A short base must be measured on the Tortugas, and an astronomical station for azimuth observations be made there. In that group, the triangulation from the Logger-Head base will furnish a proper side of nearly five miles, and a distance of four miles may be had at the Marquesas from the eastern triangulation.

The base lines, referred to in my last annual report, have been cleared with some labor, levelled, and well prepared for measurement. (See sketches F No. 3, and F No. 4, report of 1850.)

and

In the triangulation of this season 30 signals have been put up, determined from 11 stations, forming 41 triangles, and embracing the whole of the harbor, with all four channels of the Cedar keys, and the islands themselves. A small preliminary base of about two miles was measured on Way key by chaining, and the ends, as well as all the stations of the triangulation, secured by posts. Six of the primary stations have been marked by stone monuments. The area covered by the triangulation exceeds 40 square miles; 96 angles were observed, usually by six repetitions direct and six reversed, sometimes by more frequent repetition.

Assistant Gerdes and party have also carried on their office-workcomputation of results-during the season.

Secondary triangulation of keys and reef.-This part of the work was conducted by assistant J. E. Hilgard from December 20, 1850, to February 1, 1851, and from that time up to June 3d, (when the season was at an end,) by assistant L. F. Pourtales. The field of operations embraced Boca Chica, Key West, and the Marquesas, during the former portion of the season; and during the latter, Key Biscayne bay, including Soldier key, Ragged keys, Elliott's key, and Rubicon Point, with the main land from Shoal Point (twenty-five and a half miles, by the shoreline, southward from the mouth of the Miami river) to Turkey Point. (See sketches F No. 1 and F No. 2.) Mr. Hilgard gives the following statistics of his work: 9 stations occupied; 18 ditto observed upon; 100 angles, each the mean of six repetitions, observed (with the eight-inch Gambey theodolite, C. S. No. 24;) area of the work, 78 square miles. Mr. Pourtales reports 3 stations occupied; 18 points observed upon; 497 sets of measures of angles made; area of work, 70 square miles.

His comparatively small progress is accounted for by difficulties occasioned by the prevailing trade-winds, and by the uncertainty often arising from lateral refraction—the lines passing over shoal water, which rendered it necessary to repeat the observations frequently.

Much time has been occupied and great difficulties encountered, necessarily, in the erection of signals. Where it is necessary to insert them in the rock of the reefs the patent screw-pile of Mitchell has not yet been successfully applied; but it is hoped that, before the next season for observation, the object will have been accomplished by the addition of a heavy capstan-head applied to the screw-pile, and a tripod to steady it.

Should this apparatus not prove effective, the object may be attained, as suggested by Mr. Pourtales, by a modified application of an arrange

ment already used by Major Bache, of the United States topographical engineers, consisting of an iron disk sunk into a bed, and having a socket in which to fasten a signal-pole.

Lieutenant James Totten, United States army, joined this party in the latter part of the season, with a view to qualify himself to take charge of its operations during the next.

Topography.-During the first five months of the current year, a party, under charge of assistant Hull Adams, assisted by Mr. R. M. Bache, was engaged in the survey of the keys and shore-line indicated in the accompanying sketch F, No. 2.

Sheet No. 4 contains Man key, Woman key, Mule key, Boca Grande, and the Marquesas.

This latter group embraces 9 distinct keys, has an area of 12 square miles, and a shore-line of 30 miles.

Sheet No. 5 contains Bahia Honda and seventeen smaller keys.

Sheet No. 6 contains Key Biscayne, Virginia key, and 25 miles of the shore of the main land, from the mouth of Miami river southward to Shoal Point. It may be useful to mention two points on this shore where excellent fresh water can be procured: one, the "Punch Bowl," is about two miles, and the other, called "the Hunting Grounds," about twelve miles, below the mouth of Miami river.

The whole extent of shore-line included in the season's work is 139 miles, and the amount of area 186 square miles. Boca Grande, Woman key, and Virginia key, were surveyed by Mr. R. M. Bache.

The character of these keys is so little known that it may be well to quote the general description given by Mr. Adams:

There is very little variety in the character of the topography of the keys surveyed during the past season. They are generally covered with a thick growth of mangrove, which extends quite to the water's edge. On the parts of the keys towards the reef, however, there is sometimes a good beach; but the entire inner side is of mangrove, with a very deep, thick mud along the edge. Upon the southern and eastern sides of the Marquesas there is a very good beach, which extends for about six miles, interrupted only by one inlet, of one hundred and fifty metres breadth. Key Biscayne has a fine wide beach on the eastern side, nearly four miles in length, of Atlantic sand, which is quite different from the shells and ground coral (Gulf saud) of the other keys. On the western side there is a very high growth of mangroves, and the character of the mud seems to be much the same as that about the other keys. This mangrove thicket extends nearly half across the island, for two-thirds of its entire length. The other part is covered with a thick palmetto scrub. The shore of the main land, commencing at Miami river, has a sort of hammock growth of about a quarter of a mile in width for three miles; after which, for about five miles, there is a prairie growth which extends back to the commencement of the pines. This pine land extends, in many places, to the everglades. For the next four miles the hammocks increase in width, interspersed with swamps and prairie grass. A narrow ledge of rocks, about ten feet in height and covered with trees, is a distinguishing feature of the first three or four miles below the mouth of the Miami river."

Hydrography. The hydrographic operations were delayed by circum

stances beyond my control, and were not commenced until January. Lieutenant John Rodgers, United States navy, assistant in the coast survey, has been in charge of the party, and commanding the Coast Survey steamer Hetzel, employed in the work.

for

The minute survey, now completed, of the harbor and approaches of Key West, has determined the valuable result, that safe passage vessels of any class into that port may be permanently indicated. As remarked by Lieutenant Rodgers, the difficulties of approach for large vessels would at first appear insuperable. He points out positions in which a vessel may have three fathoms on one side, and six on the other-may run over a point with seventeen feet water on it, and have thirty-six feet by both leads. The profile view (sketch F, No. 5) is furnished by him to illustrate this extreme irregularity of the coral formation.

The drawing of the entire chart of Key West (see sketch F, No. 5) is furnished for publication, and its engraving has been commenced.

The general depth on the spots in the harbor varies from ten to twenty feet, and is, in their immediate vicinity, from twenty-five to thirty-six feet. Lieutenant Rodgers estimates that a safe channel for the largest ships may be marked out at an expense of about $2,000, by fixed buoys, to be secured with heavy chains and Mitchell's mooring-screws; and observes that shifting buoys would be worse than useless-a snare. The value of this harbor is well urged by him, as follows:

"All the ocean-commerce of the valley of the Mississippi keeps within a few miles of Key West. The harbor is important as a place of refuge and repair, and this value grows with the commerce of the

West.

"In time of war with a maritime power, its importance will be greatly enhanced, and its waters will be white with every class of sail, either seeking protection under its guns, or lying in wait for an enemy."

The important passage of Boca Grande (from the Gulf of Florida into the Gulf of Mexico, used when the winds are light and the Gulf Stream current strong) has been surveyed, and the chart is nearly completed. It has been ascertained that thirteen feet can be carried over the bar.

A survey has been made of Mosquito harbor, which will be further mentioned in its proper connexion, (see sketch F, No. 3.) For a brief account of the inlet, its past condition, and recent changes, see Appendix No. 32, containing extracts from Lieutenant Commanding Rodgers' report. Soundings were attempted across the Gulf Stream, from Key West to Havana, but unfortunately arrested by the loss of the line; and in returning from Havana, the new method of sounding with twine could not be tried on account of bad weather. The experiments, however, were not without interest. Lieutenant Rodgers states: "We then ran thirty minutes S. S. W. by compass, or three miles from the last position, and in the same place (except drift) the following casts were taken:

370 fathoms, no bottom, white wax on end of lead untouched.

"In reeling up, the line parted 245 fathoms from the surface of the

water.

"At 2,977 fathoms, I thought that the lead had not reached bottom, because I could perceive no abatement in the velocity of the line running through Burt's patent sounding nipper. Whether the lead did reach bottom at that depth would have been shown by the wax arming upon regaining the lead; unfortunately it was not regained.

"After leaving Sand key, the vessel was, by astronomical observations, first set to the westward of her course, and afterwards to the eastward of it. The lead-line seemed not to partake of this drift, since the vessel was first set to the westward of the line; and to the eastward of it when the current farther from the shore had become easterly. These facts lead me to the inference (which the difference of the temperature shown between the surface water and the deep-sea soundings may seem to corroborate) that the current does not run at such great depths as the lead reached."

For the season, from January 4 to June 17, 1851, the work of the steamer and boats amounted to 2,492 miles of soundings, for which there were 95,332 casts made of the lead, and 4,120 angles measured for position.

Pending the delay in the arrival of the vessel assigned to Professor Agassiz for transportation, Lieutenant Commanding Rodgers afforded his aid and the use of the steamer Hetzel, for the exploration (before mentioned) conducted by that gentleman.

Light-houses, buoys, &c.-Assistant Gerdes' examination (Appendix No. 31) has confirmed the suggestion of Lieutenant Commanding James Alden, United States navy, reported last year, that Sea-horse key is the only suitable site for a light-house, which was appropriated for by the last Congress, to be placed somewhere in the approach of Cedar Keys. It should be placed at a sufficient elevation (for which a spot elevated 45 feet is designated by Mr. Gerdes) to show over the whole Sea-horse reef. That reef has been found to extend in reality about fifteen miles, although its length has hitherto been laid down on the charts as about seven.

Mr. Gerdes adds: "There ought to be a buoy placed on the point of this reef, by all means. In foggy weather the Sea-horse (key) is invisible from thence, and the reef is very difficult to distinguish, as the water outside deepens only gradually, and retains the same color. A screw-pile, (disk-screw,) with a barrel, could be inserted easily, the reef being only of quicksand, and a sort of coral sand."

Lieutenant Commanding John Rodgers, United States navy, assistant in the coast survey, has surveyed the Mosquito inlet with reference to the placing of buoys. For his report see Appendix No. 33, and sketch F, No. 3. In the same report he discusses the subject of communication between the local pilot and the vessel, in weather when boats cannot cross the bar. Local conventional systems of signals have been adopted for this purpose in some places, but it is highly desirable that a uniform method should be adopted by authority, for all the barred harbors of the United States; such a plan forms part of his report, and it is respectfully commended for consideration, (see Appendix No. 33.) Assistant Gerdes recommends very strongly-and his recommendation

is concurred in by Lieutenant Commanding Rodgers-that a lighted beacon or a light-house be placed on Rebecca shoal, between the Marquesas and Tortugas. The passage in which this shoal or reef lies is a thoroughfare for vessels bound to ports in the Gulf of Mexico; the shoals are therefore dangerous, and at present are not marked. For the letters of Lieutenant Commanding Rodgers and Mr. Gerdes on this subject, see Appendix No. 34.

SECTION VIII.-FROM MOBILE POINT TO VERMILION BAY, INCLUDING THE COAST OF ALABAMA, MISSISSIPPI, AND PART OF LOUISIANA.

(Sketch H.)

The operations of this section have been on the same general scale as hitherto, the greater proportion of the means being, however, devoted to the hydrography, which is proportionably less advanced than the other parts of the work. One secondary triangulation party and one topographical party have been employed in extending the work westward, over Mississippi sound and Lake Borgne; and a hydrographic party, having an efficient steam-vessel (the Coast Survey steamer Walker) and other facilities, have been occupied in off-shore work south of Dauphin and Petit Bois islands, and in-shore work north of the same islands, and in special examinations of Pass Christian harbor and the mouths of the Mississippi.

The secondary triangulation of the previous year was necessarily in part repeated, and a new connexion made with the primary work; which, with the character of the season, has rendered the progress less than was expected. The triangulation of the Bay of Biloxi and of St. Louis bay was completed. The topography has well advanced to include both sides of Pass Christian.

A steam-launch, constructed by Mr. J. G. Young, engineer United States navy, under the immediate direction of Lieutenant Commanding Jas. Alden, for in-shore and harbor work in this section, was unfortunately lost in a storm off the Chandeleur islands in May, so that the party had but little aid from her services.

The discussion of the tidal observations at Cat island has been continued under my immediate direction, and the results deduced have been found of considerable interest in a practical and scientific point of view. Persons who had longest observed these tides believed that they were without regularity, and navigators generally who had attended to them, supposed that they resulted from the action of the wind. I have shown, on the contrary, that the results are strictly referable to law, and do not depend upon the wind, except as a modifying cause. The first results deduced are given in a paper presented, by authority of the Treasury Department, to the American Association for the Advancement of Science, at their meeting in August, 1850, which, with some revision, is transferred to the Appendix No. 7. It is not a little curious that a case of the single-day tides, to which these belong, (having, as a general rule, but one high and one low water in the twenty-four hours) should be among the earliest on record, having been examined by Newton in his original discussion of the tides in connexion with the theory of gravitation. The phenomenon met no further development until recently