4th. The weight of newspapers must be taken or determined when they are in a dry state.

5th. Postmasters are not entitled to receive newspapers free of postage under their franking privilege.

6th. Payment in advance does not entitle the party paying to any deduction from the above rates..

NOTE. For each additional ounce, or fractional part of an ounce, beyond the ten ounces embraced in this table, an additional rate must be charged.

RATES OF POSTAGE TO BE CHARGED, AFTER JUNE 30, 1851, UPON ALL TRANSIENT NEWSPAPERS, AND EVERY OTHER DESCRIPTION OF PRINTED MATTER, EXCEPT NEWSPAPERS AND PERIODICALS PUBLISHED AT INTERVALS NOT EXCEEDING THREE MONTHS, AND SENT FROM THE OFFICE OF PUBLICATION TO ACTUAL AND BONA FIDE SUBSCRIBERS.

1.-On every transient newspaper, unsealed circular, handbill, engraving, pamphlet, periodical, magazine, book, and every other description of printed matter, the above rates must in all cases be prepaid, according to the weight.

2. Whenever any printed matter on which the postage is required to be prepaid, shall, through the inattention of Postmasters, or otherwise, be sent without prepayment, the same shall be charged with double the above rates.

3.—Bound books, and parcels of printed matter, not weighing over 32 ounces, shall be deemed mailable matter.

Periodicals published at intervals, not exceeding three months, and sent from the office of publication to actual and bona fide subscribers, are to be charged with onehalf the rates mentioned in the last above table, and prepayment of a quarter's postage thereon must in all cases be required. Periodicals published at intervals of more than three months are charged with the full rate, which must be prepaid.

NOTE. In case there is on, or in any newspaper, periodical, pamphlet, or other printed matter, or paper connected therewith, any manuscript of any kind by which information shall be asked for, or communicated in writing, or by marks or signs, the said newspaper, periodical, pamphlet, or other printed matter becomes subject to letter postage; and it is the duty of the Postmaster to remove the wrappers and envelopes from all printed matter, and pamphlets not charged with letter postage, for the purpose of ascertaining whether there is upon or connected with any such printed matter, or in such package, any matter or thing which would authorize or require the charge of a higher rate of postage thereon.

POST OFFICE DEPARTMENT, June 14, 1851.

N. K. HALL, Postmaster General.

ANNUAL REPORT OF THE BALTIMORE BOARD OF TRADE.

The second Annual Report of the Baltimore Board of Trade for the year ending, October 1st, 1851, which we here subjoin, is a model of brevity and comprehensiveness: and illustrates the importance of such associations to Commercial cities, when under the management of intelligent and liberal merchants:

The Legislature of Maryland not having been in session since the date of last Report, such subjects as had been brought to the notice of that honorable body when last convened, and were not then acted upon, remain of course still in abeyance, and must be referred anew to the Legislature shortly to assemble at Annapolis; and they will doubtless receive the proper attention in that respect from the officers the Association are about to elect for the ensuing year.

In consequence of the "short session" of Congress last winter, several matters of much importance to the trade of the city, although ably and energetically urged by our late representative at Washington, were not acted upon, and renewed efforts must be made hereafter to secure the aid of the federal government in reference to such objects as are of a national character, in fostering and facilitating the Commerce of Baltimore. Some questions, however, in regard to which the Board have exerted themselves, were favorably disposed of; among them may be mentioned the passage of the bill known as the "Cheap Postage" law, which, although it does not go to the extent recommended, of an uniform rate of two cents per half ounce prepaid, may still be regarded as a great concession to the interests and convenience of the entire community.

For the erection of a Light-House on the seven foot knoll, near the junction of the Patapsco River and Chesapeake Bay, the sum of seventeen thousand dollars was appropriated by the bill of 3d March, 1851, leaving at the command of the Secretary of the Treasury twenty-seven thousand dollars for the Light-House in question; thus far a commencement has not been made, but it is to be hoped that ere long the work may be accomplished.

Another strenuous but unsuccessful effort has been made before the City Councils to have the enactments upon the inspection and storage of Beef and Pork altered and amended, so as to leave untrammeled this important and growing branch of business; the attempt, however, to place this market upon a fair basis of competition with those North and East of us will be again resumed, and as a large majority of dealers in Provisions, whether buyers or sellers, packers or exporters, concur in recommending the proposed alterations in the enactments, they will probably ultimately be granted. It having been represented that "Through Tickets" from Charleston to New York were granted on the various works constituting the main line of travel with a discrimination unfavorable to Baltimore, a correspondence between the appropriate committee and the President of the Baltimore & Ohio Railroad Company ensued, and by the exertions of that gentleman with the representatives of connecting links, the tariff of rates of passage has been equitably adjusted.

Memorials, numerously signed, addressed to the Postmaster General and representing the necessity of enlarged accommodations for the reception and distribution of the Mails, were brought to the consideration of the Board. A special committee appointed for the purpose examined the bearings of the subject, and recommended the removal of the Post Office to the Exchange Building, where it now is, and the change apparently meets the approval of the public.

It having been understood that some idea was entertained at the Department to suspend, or even entirely withdraw, the Express or Special Mail Train westwardly, a correspondence was opened with the Chambers of Commerce of Wheeling, Pittsburg, Cincinnati, Louisville and St. Louis, asking their co-operation in remonstrating against such a proceeding; it was cheerfully accorded, and the Postmaster General appears favorably to have regarded the representations on the subject, as we are still in the enjoyment of this second mail and passenger train between Boston and Cincinnati six days in the week. It would appear that, by avoiding the detention of some hours at Petersburg, Virginia, the Great Southern Mail might reach here sufficiently early to admit of reply the same evening, which is not at present the case, and twenty-four hours are thus lost to correspondence; it is hoped this unnecessary delay will soon be done away with.

A communication was received from the Chamber of Commerce of New Orleans relative to "the causes of the explosisn of steam-boilers, and the measures deemed necessary for their prevention," and invoking the assistance of this Board in the matter;

our representative in Congress will be requested to urge the passage of such laws as in his opinion may remedy the evils named in that document.

The attention of the Board has been given to various other subjects of more or less importance to the commercial community, but it is not deemed requisite to enter into further details on this occasion; before, however, concluding this Report, it is thought a duty once more to refer to the imperative necessity of some action towards deepening the Ship-Channel, in order to maintain the advantages of Baltimore as a maritime port. It is well known that each year the average size of vessels built is increased in tonnage and draught of water, likewise that deeply laden ships, entering and leaving our harbor, frequently ground, to the manifest danger of vessel and cargo, to say nothing of loss by detention; it is no isolated interest that thus suffers, every citizen is concerned directly or indirectly, and in the opinion of this Board prompt and efficient steps should be taken to remove such impediments as may exist. If an appropriation cannot be had from the General Government, surely the State and City can be induced to unite in furnishing the sum necessary to do away with such obstructions in the Ship-Channel as interfere with the egress or ingress of mercantile ships of the largest class.

The accompanying statement of the Treasurer shews that he has in cash $181 01; the association is also possessed of 17 shares of Stock of Merchants' Bank of Baltimore, and $700 Maryland State 6 per cent Stock.

All of which is respectfully submitted. By order of the Board of Directors.

JNO. C. BRUNE, Prest.

NAUTICAL INTELLIGENCE.

VARIATION OF THE MAGNETIC NEEDLE.

Something like ten or twelve years since I addressed a letter to one of the professors of Middlebury College, stating the expediency of adopting a course of experiments upon the magnetic variation, requesting his co-operation in bringing the subject before the public. It appeared to meet his approbation, and he gave the communication publicity in the Middlebury and Albany papers. But as he left the college soon after, nothing further transpired on the subject. And although it is a science almost totally neglected by surveyors in this State, it can but be viewed as one of no ordinary importance.

That observations should be annually made by every practicing surveyor no one can doubt. As the boundaries of lands are usually described according to the course indicated by the needle, and as there are no rules by which its variation can be ascertained for any interval of time, according to which such bounds can be retraced where the land marks have been obliterated, it therefore appears of importance that surveyors should, from year to year, ascertain at different places the true variation of the needle, and note it in their surveys, which would prevent much litigation.

The phenomena of the magnetic attraction have for many centuries engaged the attention of philosophers, not only from the obscurity in which it is involved, but from the importance of the subject. It would, however, seem that no very satisfactory conclusions as to the main cause any one has as yet arrived at, though I believe it to be generally conceded that it lies hid on the earth and near the poles. Although considerable affinity is discoverable between the the electric and magnetic powers, yet in what manner electricity acts in producing magnetism, we do not learn, as it is, as yet, not distinctly known, but continues to be one of those hidden mysteries that defies the sagacity of the most scrutinizing genius.

The magnetic needle affects the situation of being in direction from south to north, but there are accidental causes capable of deranging the direction. You have only to present the point of some well-tempered steel, and it will immediately leave its direction. Iron alone does not produce the effect, as a compass may be used to advantage even in iron mines.

Every practical surveyor well knows that the magnetic needle does not always point due north, and that the variation is different not only in different places, but at the same place at different periods of time, but is the same to all magnetic bodies at the same place.

Many curious experiments have been made, and a multitude of theories advanced to account for its surprising qualities. Newton, Halley, Kepler, Euler, Churchman, and a

number of later date, have each adopted different theories and conclusions, as to those attractive bodies, but generally concede that the cause is within the bowels of the earth, and near the poles. The first discovery of the variation of the magnetic needle was in the year 1492, by Columbus, in his first voyage to America, but it was not until the year 1580, that the subject was carefully studied and thoroughly investigated at London, when the variation was found to be 11° 50' E., and in 1620 but 6° E., and in 1634, 4° 5′ E., but in the year 1660 the pole pointed due north.

In 1672, it was found to be 2° 30′ W., and in 1692, it was 6° W. In Paris the needle pointed due north about nine years after that at London.

At present through Europe, Africa, and a part of Asia, the declination is to the west, but advancing eastward at the rate of one degree in 19 years.

It is, however, ascertained that the declination never exceeds 15° on or near the Equator, but increasing towards the poles to 60°.

Another surprising quality discoverable in the needle, is its inclination or dipping, that is, the magnetic power produces a double effect on needles.

This, as well as the declination, deserves to be everywhere carefully observed and noted; in London I believe the inclination to be about 68°. In July 1820, Mr. Sabine observed the inclination of the needle at Melville Island, in lat. N. 75°, W. lon. 110°, found it to be 88° 43′ 5′′.

The following observations and calculations were made chiefly in the State of Vermont; partly by Dr. Samuel Williams, the more recent by the writer:

In the year 1785, the declination of the needle at Montreal was 8° 24′ W., and at Quebec, 12° 50′; in 1794, 12° 20'. On the north line of Vermont in the year 1785, the variation was 7° 40′ W., and at Missisque bay the same year, 10° 10' W., and in 1828, but 8° 50'. In Pownall in 1786, the variation was 5° 50′ W., and in 1828, but 4° 10'. At Rutland in the year 1789, the variation was 7° 5' W., and in 1818, 6° 10', and in 1828, 5° 40′, and in 1848, 4° 39'. At Burlington in the year 1793, the variation was 7° 30', and in 1828, but 6° 45′ W. At Brandon in the year 1820, the variation was 5° 20', and in 1830, 4° 50' W. At Pittsfield, Vermont, in the year 1825, the needle varied 6° 5′ W., in 1826, 6° 2′, in 1830, 5° 50', and in 1836, 5° 34' West. In New Haven in 1820, the variation was 4° 25' 25", according to Professor Fisher, who supposes the annual variation to be 2' 45" eastward, but from personal observations made by the writer, the variation is found to be something over three seconds.

From the above view it is no way surprising that so much litigation has arisen in consequence of surveys being made at different periods of time with little or no attention to the annual variation, and unless there is some method adopted to make it the duty of every practicing surveyor to ascertain from time to time the true variation, and note it as before observed, different cources will be run, and litigation continue.

Mr. Dewit truly remarks, "that in years past, a rule has been prescribed for obtaining an approximate meridian supposed sufficient for common purposes, that is, to take the direction of the Pole star when in the same vertical line with Alioth, which is the first star in the tail of the Great Bear." This rule was once correct, but it is more than a century since, that the interval between the time when these two stars are in the same vertical, and the time when the Pole star is in the meridian, has been gradually increasing, on account of the annual increase of the right ascension of the Pole star than of Alioth. According to Blunt's table for the year 1805, we find that in lat. 42° 30′ the elongation of the Pole star was 2° 20′ 51′′; and in the year 1837, I found it to be but 20 72" in the same lat.; and in 1839, 2° 6' 7", and in 1840, 2° 5′ 41′′. In lat. 42° in the year 1837, 2° 5' 52", in 1848, 2° 5' 26", and in 1839, 2° 5′ 0′′, and in the year 1840, 2° 4' 34".

These annual variations will show the importance of a strict attention to time and place of observation.

Various methods have been instituted to ascertain an accurate variation, but with much inaccuracy, and I apprehend that the main difficulty is in the finding a true meridian. The following is therefore recommended, being simple and the least subject to error: viz., by measuring the angle formed between the magnetic meridian and a line formed by the Pole star when on the meridian.

But in this process it is necessary to know that this star is due north but twice in twenty-four hours. The time may be found by observing when the star Alioth, and the star Gamma, and the Pole star, are vertical; but when in a horizontal position, is at its greatest elongation on the side of Gamma. In order, therefore, to find a true meridian from the star, its declination must be calculated for the degree of latitude

where the observation is made. This may be found by the following proposition:As the cosine of latitude is to radius, so is the sine of declination to the sine of elongation. The calculation may be also made as in the 6th and 7th cases laid down in Euclid's Spherical Trigonometry. According to the above proposition, the North star, in lat. 43° 30', January 1829, was in its declination 87° 47′ 37′′, and its elongation 2° 18′ 22′′, and increasing at the rate of 19' 59" annually in the same latitude.

In lat. 43° the same year the elongation was 2° 10' 13". From the above dates and the application of the rules given, the variation of the magnetic needle is easily found in any latitude by a properly informed surveyor.

CASTLETON, Vermont.

E. CHILD.

ROCKS AND SHOALS IN THE PACIFIC.

The following list of islands, rocks, and shoals in the Pacific are not laid down on the charts:

[In lat. 20° 30' N., lon. 152° 30′ E., lies a small sandy bank, with heavy breakers, N. E. by N. of Nooaheva.]