Imágenes de páginas
PDF
EPUB

A record of the tides, day and night, was kept here.

7. The removal of certain of the reefs in the dangerous passage of Hell Gate having been resolved upon by enterprising citizens of New York, it appeared desirable that the reefs should be minutely examined before and after these operations, and that the changes in the hydrography of the passage should be carefully noted. Lieutenant Washington A. Bartlett, United States navy, assistant in the coast survey, was detailed for this purpose, and made minute surveys of Way's reef, Pot Rock, the Gridiron, and Hallet's Point; the first two being made in October after the first blastings, and the second before any such operations had been attempted. For his reports, see Appendix No. 56.

To the perseverance and urgency of Eben Meriam, esq., of Brooklyn, the liberality of Henry Grinnell, esq., and others, of New York, and the skill of Mr. Maillefert in submarine blasting, the city of New York will owe an undertaking which has already so far succeeded as to render it almost certain that the dangers of this formidable passage may be essentially diminished, if not entirely removed.

The Coast Survey chart of Hell Gate, and its approaches, will be modified in accordance with the changes which may be developed, and the first edition will serve as a historical record of no small value and interest.

8. Lieutenant Commanding S. Swartwout, United States navy, assistant in the coast survey, was detached with the brig Washington, for a short time, to make the hydrographic survey of the vicinity of Cox's ledge, which is some twenty-one miles from Block island. Thirty-four miles of soundings were run over, and sixty-eight soundings procured, in from 16 to 24 fathoms water.

9. Valuable reports have been received from the Hon. H. C. Murphy, of Brooklyn, on the names upon the Coast Survey map No. 1, of Long Island sound, including both shores, giving the orthography and derivation of some 270 names. This is in continuation of his reports on sheets Nos. 2 and 3 of the same map.

10. In my report of 1849, I noticed the operations for determining, by telegraph, the difference of longitude of Seaton station (Washington) and Western Reserve College, with a view to render available for the longitude of one of the Coast Survey stations the elaborate series of moon-culminations observed by Professor Loomis, (now of New York University,) between the years 1838 and 1844.

It was found impracticable to transmit the signals to and from Washington, but from and to Philadelphia the telegraphic connexion with Hudson was complete on several nights during the time set apart for the work.

The telegraph and other observations have since been reduced under the direction of assistant S. C. Walker, who deduces from them the longitude of the observatory of Western Reserve College, Hudson, Ohio, west of the High School observatory, Philadelphia, 25m. 5.70s. 11. The results of the observations for difference of longitude between the Seaton station, Washington, and the High School observatory, Philadelphia, made in the summer of 1849, have been computed by assistant S. C. Walker, or under his immediate direction. They

consisted of the exchange of chronometer signals and of star signals, and, with the weights assigned to the different classes of observations by Mr. Walker, give the difference of longitude 7m. 20.90s.

SECTION III.-FROM CAPE HENLOPEN TO CAPE HENRY, INCLUDING

THE COAST OF DELAWARE, MARYLAND, AND PART OF VIRGINIA.

(Sketch C.)

Seven land parties have been employed in this section during the season, either in whole or in part; one, a double hydrographic party throughout, and another for a portion of the season. The astronomical observations for connexion of the Capitol with the coast have been finished; and a scheme devised, after, reconnaissance, for connexion of the Chesapeake with the seacoast triangulation. The primary triangulation of the Chesapeake has been extended from the limits of last year towards the Capes, at which some preliminary determinations have been made, embracing in area more than half of all that remained to be completed at the time of my last year's report.

The system of connexion, by telegraph, of Washington with distant points of the coast for difference of longitude, has been prosecuted. The secondary triangulation has extended along the peninsula of Virginia, from Cedar island to Rogue's island on the Atlantic coast. The topography of the Patapsco river has been revised. Other topographical surveys have covered part of the seacoast of the peninsula of Maryland and Virginia; and both shores, with the islands of the Chesapeake bay, in that portion including Pocomoke sound on the east, and lying between the Potomac and Rappahannock rivers on the west.

The hydrographic operations have advanced along the outer shore of Virginia, from a point in Maryland near the line, to within about forty miles of Cape Charles, including important shoals on the coast, and extending seaward a distance of about eleven miles. They have likewise been continued in Chesapeake bay; and a special survey, of which the results are made public in a sketch herewith, has furnished valuable information in regard to the approaches (middle ground) of that bay.

Astronomical observations, &c.-Causten's station, near Georgetown, D. C., was occupied during May and June by my party, for the determination of latitude and azimuth. Magnetic observations were also made, and an angle in one of the secondary triangles measured, to furnish a base for continuing the survey down the Potomac. The observations were made, under my immediate direction, by sub-assistant George W. Dean; and the following table shows their number and kind, and the instruments with which they were made:

Observations for

Time.-Transit No. 4, (Troughton & Simms,) 102 observations on 12 pairs of stars.

Latitude.-Zenith sector, C. S. No. 1, (Troughton & Simms,) 366 ob

servations on 80 stars.

Azimuth.-30-inch theodolite, C. S. No. 1, (Troughton & Simms,) 130 observations on Polaris.

Magnetic. Variation, declinometer C. S. No. 1, by Jones, 77 observations on five different days; intensity, declinometer C. S. No. 1, by

Jones, 3 sets on three different days; dip, 10-inch dip-circle by Gambey, 3 sets on two different days.

The stars used in the latitude observations were selected from the Greenwich twelve-year catalogue, for which we are indebted to Professor Airy.

The micrometers of the zenith sector were compared with the divisions on the limb of the instrument, giving the same resulting mean value as was obtained last year.

The azimuths were in part observed at elongation and in part at culmination, by the method referred to in my last report. They were referred to a mark placed nearly in the line to Soper's station, (Montgomery county, Maryland,) and the angle between the mark and the signal at Soper's was measured by the micrometer.

During the months of July and August, moon culminations were observed at the Seaton station by Mr. J. C. Langton, under the direction of assistant S. C. Walker, by the galvanic method. In his annual report, Mr. Walker brings up the results of the investigation of the rate of transmission of the signals through telegraph lines (galvanic wave time) to the present date. "The results of our experience to October, 1850, were then stated to be

"1st. That the average of all our experiments to that time indicated a velocity of propagation of the galvanic wave of 15,400 miles per second in the iron wires of a telegraph line.

"2d. That the velocity of propagation through the ground appeared to be less than two-thirds of the velocity in the iron wires."

During the year, three new results have been added to the foregoing, two of them obtained incidentally while determining differences of longitude. The first, between the Seaton station, Washington, and Portsmouth, Virginia, when the distance through the wires was 268 miles, and through the ground 180 miles; the second, between Charleston, South Carolina, and Savannah, Georgia, passing through Augusta, the length of the circuit of telegraph wires being in this 447 miles. In the third, from Cincinnati and back, the circuit consisted of 840 miles of iron wire, there being no ground connexion. From a comparison of all the results, Mr. Walker draws the conclusion that the time of traversing 15,461 miles is one second. As these experiments have attracted much attention, at home and abroad, among scientific men, and afford the data for examination of questions in regard to the propagation of the galvanic influence which are of high interest, I have placed Mr. Walker's report, entire, in the Appendix No. 25.

The telegraphic operations for determining difference of longitude between Seaton station and Forbes' point, North Carolina, and the general arragement for those between Charleston and Savannah, were under the charge of assistant Sears C. Walker. An account of these has been given, in the proper place, in Sections IV and V. In addition, he has been industriously occupied in reducing and superintending the reduction of his telegraph results, and of the longitude observations of

others.

This is termed by Mr. Airy, the Astronomer Royal of Great Britain, the "American method."

During the year, the telegraph work of 1846 and 1847 has been discussed by him, using the best values for personal equations, and he now reports the discussion as final.

The work of 1848, between Washington, New York, and Cambridge, has also been finally discussed, and the geodetic corrections applied. That between Philadelphia and Cincinnati, in 1849, Philadelphia and Hudson, and Seaton station and Charleston, in 1850, is also reported as complete; and the results of observations of moon culminations and occultations at Hudson, by Professor Loomis, have been used in determining the longitude of Cambridge.

The following extracts from Mr. Walker's annual report will show the progress of the computations of moon culminations, &c., under his charge:

"In the course of the last year additional corresponding moon culminations have been found for Mr. Hilgard's work at Sand key, Florida, Section VI, and reduced and reported.

"The observations of assistant Davidson, at Point Conception, Upper California, Section X, of moon culminations, have been reduced and compared with those made at Philadelphia and Cambridge. The longitude of Point Conception has been obtained and reported."

"No progress has been made in the further reduction of the collection of occultations and eclipses of the coast survey. I recommend that these be compared and reduced, with the recent improvements in the lunar theory, in particular including that of Mr. Myers Fisher Longstreth, of Philadelphia.

"The work of reducing the moon culminations of the coast survey has been continued by Professor Pendleton, with the assistance of Professor Yulee and Mr. Keber. Nearly all the work at Hudson, Ohio, has been revised; and in instances where Lieutenant Gilliss, United States navy, and Professor Loomis differ, a triplicate computation is made. A part of the work of Lieutenant Gilliss, United States navy, has been submitted to a duplicate computation.

"Professor Pendleton's work has been, thus far, limited to the years. 1841 and 1846."

"Corresponding observations of moon culminations only have been hitherto reduced, in consequence of the state of the lunar theory. The recent improvements in that theory by Hansen, Airy, Longstreth, and Peirce, make it desirable to compare all our observed moon culminations with theory alone. Where corresponding observations are available, it is well to compare also with observation. It will serve as a test of the recent improvements, and may, perhaps, serve to throw some light on the discrepancy which now exists between the longitude by moon culminations and by chronometers.

"We are relieved from the necessity of reducing the numerous observations of the recent solar eclipse, by the amateur computations of Professor Peirce."

Referring to the results of the various chronometer expeditions, reported since 1844 by Mr. Bond, director of Harvard observatory,

including the special expeditions of the Coast Survey to Liverpool, Professor Walker remarks:

"I am convinced that there is no possible interpretation of them which will place the results below his reported value, 4h. 44m. 30.1s., for the longitude of Harvard observatory from Greenwich observatory. "The longitude of Harvard observatory, by moon culminations at Harvard observatory, Dorchester, Hudson, Wilkes' observatory, and the Washington observatory, comes out very uniformly about 4h. 44m. 28.48., leaving a discrepancy below the results by chronometers of 2.78. This discrepancy will, I hope, be explained in time by persevering efforts." Mr. Walker gives an abstract of his report on longitude, which will be found in Appendix No. 26.

A meteorological register was kept at Causten's station, by Mr. B. F. West, recording, for thirty-two days, observations for temperature of the air, moisture, atmospheric pressure, and the force, amount, and direction of winds and clouds.

Reconnaissance, primary triangulation, &c.-Major Henry Prince, United States army, assistant in the coast survey, made, during the earlier part of the season embraced in this report, an extended reconnaissance of the eastern shore of Maryland and Virginia-the purpose of which was to arrive at a mode of connecting the secondary triangulation of the Atlantic shore with that of the Chesapeake. The results of his examination show great difficulties in making the connexion high up the peninsula, for want of suitable natural elevations for signals, and on account of the heavy cutting involved in the use of any station on the main land. It therefore appears advisable, as it is reported practicable, to make the connexion by observing from island stations on either side-upon Drummondtown or some point where the peninsula is narrow-or to postpone the connexion until Cape Charles is reached by the Chesapeake and by the outside triangulations.

From this reconnaissance, Major Prince passed to that of Pamplico sound and others of importance.

Assistant Edmund Blunt, with a party for primary triangulation, was engaged in the lower part of Chesapeake bay for the greater part of three months, June to August; afterwards proceeding north for the triangulation of the Hudson river, and returning to this section in November. Mr. Blunt's operations in this section were the observations for triangulation and the reconnaissances appropriate to form a judicious scheme, and to establish points for hydrographical and topographical purposes. The triangulation of the season extends from Wolf Trap station (intermediate between York and Rappahannock rivers) south towards the entrance of Chesapeake bay, at which some preliminary determinations were made. A reference to the general sketch C, will show that it has adopted as primary points the principal capes and light-houses. Care was taken in all cases to mark the points used in a permanent manner for future reference.

The statistics of Mr. Blunt show 18 primary angles measured, and 67 others-in all, 136 series of angles were measured by 1,478 observations. The season's work covered an area of 400 square miles.

Secondary triangulation.-Assistant John Farley has continued this portion of the work along the outer coast of Virginia. His party, en

« AnteriorContinuar »