1815.] Mr. Court on the Fate of Roentgen. the next day until they reached the, River Tansif, where Mr. Roentgen sunk his European clothes in the river, and pat on the Moorish dress; and he then pursued his journey, accompanied only by the renegade. They were provided with two good mules, a variety of beads, and other articles of merchandise; about five hundred dollars in money, and each well armed with pistols, swords, muskets, and daggers. Mr. Roentgen was also well supplied with drugs to pass as a physician when it might be necessary in the interior. He carried with him also a very fine copy of the Alcoran, on vellum, which inight be of service to him in gain ing the protection of some sheriff. At parting, Mr. Roentgen promised we should hear of him by every opportunity, if only his name, date, and place, on a bit of paper. We, however, never heard from him. When they had been gone about three weeks, it was reported here, that the renegade and a Moor were seen passing the river at Azamore, a town to the northward of this; but, it appearing so improbable that they should have taken that route, no attention was paid to the report. When Mr. Roentgen had been gone about seven weeks, accounts came from Morocco, that a Moor of the province of Shedma had been stopped, offering for sale a watch and various other articles apparently belonging to an European; and the rumour immediately went forth, that they belonged to Mr. Roentgen, who had been murdered. The governor of this place sent for the articles from Morocco, and they were all identified as having been Mr. Roentgen's, by my brother, and the watch, as one which he always wore suspended by a ribband from his neck. There was now but too much reason to suppose this unfortunate traveller had been murdered, and that within three or four days' journey of this place; but still no one suspected the renegade. We sent to Morocco, to have the examination of the Moor taken. He persisted in declaring that he found Mr. Roentgen dead, and in a very putrid state, under a tree; and that he took from his person the various articles which he had offered for sale. About seven months ago, I received intelligence that the renegade had been seen at Arzilla, a town about 300 miles to the northward, where he was working as a gardener, and that he was going to Oran to embark for Europe. Upon sending to Arzilla, however, I could not find him, or ascertain to a certainty that he had been there. A month afterwards, a Jew who came from Mequinez told me, he saw him in that city, and spoke to him; and that the renegade was very shy of speaking to him. There is, I think, little doubt but Mr. Roentgen was murdered by the man in whom he placed his entire confidence; and that man an European! The mules, the dollars, and the various articles with which the mules were loaded, were sufficient plunder, without taking the few articles from his person, which were of little value. It is probable, too, that although the wretch could murder his master when asleep. he might not have the courage to strip him afterwards. As Mr. Roentgen had taken uncon mon pains to make himself fit for undertaking such a dangerous journey as to the interior of Africa; and, as he was a young man of considerable talents and of great. perseverance of mind, it is very much to be lamented that he should have met with such an untimely end. As a number of letters have been addressed to him at my house, the writers will have them returned, on signifying their wishes to that effect. Mogadore, October 20, 1812. A. W. COURT. To the Editor of the Monthly Magazine. SIR, H AVING been engaged in a calculation of the great solar eclipse that will happen in 1820, and conceiving that some account of the same might not be uninteresting to many of your astronomical readers, I am induced to solicit a corner in your very instructive and entertaining Miscellany. This eclipse will not be total, even where it is central, for, the moon being near her apogee, or at her greatest dis tance from the earth, her apparent se. midiameter will be considerably less than tint of the sun, leaving an annulus, or ring, of light, of nearly half a digit in breadth. The annular boundary will pass very near the eastern shores of En gland and Scotland; and, on the coasts of Norfolk and Suffolk, this eclipse will be almost annular. The central tract will commerce in. latitude 81° 43′ N., longitude 1490 404 W.; passing over Mayne's Island, to the western coast of Norway, along the Ba North North Sea, and entering Germany, not far from the mouth of the Weser, crossing that country to Trieste; thence down the Gulph of Venice, into the Mediterranean Sea; and, passing near Cape Matapan and the Isle of Candia, it leaves the Mediterranean to enter Palestine: passing between Jerusalem and Gaza, it quickly enters Arabia, where it quits the earth, with the setting sun, in latitude 27° 15' N., longitude 46° 9' E. But the penumbra will first touch the earth in latitude 59° 40′ 38′′ N., longitude 91° 5' 5" W. and finally leave it in latitude 3° 20'33" N., longitude 20° 28′ E. Owing to the great northern latitude of the moon, this eclipse will not extend farther south then latitude 13° 26' S., longitude 32° 6' E. But the penumbra will pass far above the earth in the other hemisphere. At all those places where the digits eclipsed are 11, the obscuration will be as great as where it is central, for the whole of the moon will, in such case, appear upon the disc of the sun. The sun will be central eclipsed on the meri. dian, in latitude 77° 20′ 43′′ N., longitude 16° 37' 45" W. The centre of the penumbra will be 2h. 13m. in passing over the earth, and the whole duration of the general eclipse, or the time of the penumbra passing over the disc of the earth, will be rather more than five hours and a quarter. After giving this outline of the general eclipse, I shall proceed to the calculation of it for the latitude and meridian of Greenwich; but let me premise, that the places of the sun and moon are computed with the greatest care, and from the best astronomical tables. Moreover, as the accuracy of all computations regarding solar eclipses, depends entirely upon the nicery observed in obtaining the parallaxes of the moon, I have been particularly careful on this head; and, not wishing to confide in any auxiliary tables, I have computed the parallaxes from the triangles themselves; for, in the present instance, the conjunction happens so very near the nonagesin al degree, a greater exactness was required, owing to the curvature of the apparent orbit; and I have ascertained no fewer than ten points of the segment of the said orbit, which is described during the time of the visible eclipse at Greenwich, so that the beginning, middle, end, and digits eclipsed, will be found to agree with the best observations to a surprising degree of exactness. The apparent time of the true conjunc tion is September, 7d. 1h. 51m. 27.2s., at which time the true longitude of the sun and moon is 5° 14° 47′ 41" (happening only 43' 14.6" east of the nonagesimal degree) with the moon's true latitude 44/ 37.9" N. descending the horary motion of the moon in latitude 2' 41-94" and in longitude from the sun 27' 1'58"; the horizontal parallax of the moon from the sun reduced to the radius vector, for the given latitude is 53′ 40.08". Hence the longitude of the sun and moon at the visible conjunction is 5° 14° 47' 37.8"; and the apparent latitude of the moon 3' 10-73" N. At the time of the greatest obscuration, the angle of the moon's vi sible way from the sun is 16° 56′16"; and the nearest distance of their centres 3' 2.45". Now the apparent semidiameters of the sun and moon are 15′54-81" and 14′ 51-93"; hence the parts deficient are 27′44-29", and the digits eclipsed 10o 27' 30.1" on the sun's upper limb; or 17° 18' 22" to the east of the vertical point of his periphery; at the same time, the longitude of the nonagesimal is 5o 14o 20′ 23·7", and its altitude 39° 1' 18.8"; the parallax of the moon in latitude 41' 39-72" and longitude 16.647". The moon is on the nonagesimal at 1h. 55m. 145. or about 2m. 26s. after the time of the greatest obscuration at Greenwich. At the beginning of this eclipse, the apparent latitude of the moon is 12' 11-3" N., and her visible difference of longitude from the sun 28′ 17-27"; the moon's apparent semidiameter is 14/53-28", and the point of contact of the sun and moon's limbs is 49° 9' 54'3" to the west of the sun's upper limb. But, owing to the moon's decrease in latitude, and the position of the nonagesimal at the time of emersion, the apparent point where the moon's limb quits the sun is 86° 56′0′′, to the east of the zenith of his disc when the moon's apparent semidiameter is 14′ 49.88"; the apparent latitude of the moon 5' 20" and the difference of longitude 30' 17". Eclipse of the Sun at Greenwich, September 7th, 1820: apparent time P. М. 1815.] Mr. Parry, on the Principle of Bridges. may be disposed to give a geometrical construction of the same. The semidiameter of the 5 rule, page 58, that (by Mechanics) the weight of the semi-arch is to its pressure, in the direction MA, as NM is to MA, -see the figure. FI B 27 9.65 14 43.04 SHALL once more, with your perSport poter your valuable Magazine, with a few observations on a similar subject to what I have heretofore. But the author, from whom I now venture to differ in opinion, is so far my superior in physicomechanical acquirements, that it is with the utmost diffidence I enter upon the task, although, from an attentive examination of the subject, I am persuaded that I have truth to support me; and, being thus supported, I am encouraged to proceed, notwithstanding the great disparity above-mentioned. Dr. Hutton, in his Principles of Bridges, Sec. iii. Prop. x. has, as it appears to me, fallen into more than one error. For, first he lays it down as a GEL Now, with all due deference to those superior acquirements, I contend, that Mechanics will not bear him out; for a line, drawn from N to A, will not meet the angle of abutment at right angles to it, which is required it should do by Mechanics; neither will this line be in the direction of the initial pressure, for a line in that direction will be a tangent to the arch, as the line Na. Besides the line NA intersects the curve, and is a chord to part of it above A, instead of a tangent, and consequently can no-where, within the limits of the voussoir, meet a radius of curvature at right angles. But the line Na is a tangent to the curve, and consequently in the direction of the initial pressure, and the radius of curvature VB, at the point of contact, is at right angles to it; and then (by Mecha virtually the angle of abutment, which must be transferred, or supposed to be, to the pier at a, where this line intersects the vertical line IL, or face of that pier, and that intersection will be the height of the same to calculate from, as will the vertical distance frown thence to the line DN, continued = Nm, be the measure of the vertical pressure for that purpose; and from those measures, together with the area of the semi-arch = 809, the efficacious force of the arch, to overset the pier, may be obtained by the rules given in that work. Secondly, the whole resistance of the pier is there stated to be only what will arise from the multiplication of its area, into half its thickness, that is, GLXFE XEG. But, with the same respectful deference as before, I again contend, that the sum of this resistance is equal to GLX FEXEG+LG X area of semiarch; for, as the weight of the whole arch and covering must act upon the inside faces of the two piers, the weight of the semi-arch must act upon the inside face of one; and, this being admitted, I shall refer to Example the second, in the same proposition, and compare results. By the admeasurements, as there set down, the distance of the centre of gravity from D, or DN, is 33.58 feet, which answers to the tangent of 33° 15′ of the curve D A nearly, and consequently the other tangent in the direc. tion of the initial pressure being the same from the point of contact at B to N, the whole quantity of the curve to be considered as an arch, is 66° 30'. But the whole curve, from the apparent angle of abutment at A to D, is 77° 20', and 77° 20′-66° 30′= 10° 50′, a portion of the curve, which cannot be properly considered as part of the arch, in determining the thickness of the piers. It will be found by calculation, that the distance between the apparent and virtual angle of abutment, will be equal to 2.24 feet; therefore the height of the pier to calculate from, will be 18+2.24 20 24, and NM 40-2-24-37.76, =Nm. MA= 16.42, and area = 809, remaining the same. Then, from those data, and the whole height of the pier 64, its thickness may be deduced, and it will be found to be 6912 feet, little more than half the thickness of Dr. Hutton's pier, which is 13.67 feet. Notwithstanding, the efficacious force of the arch is greater by our method than by his: for by our's it is 20-24 7120-432, and by his 809×16.42 X18-5976. 40 = 809 x 16.42 37-76 Such opposite differences in cause and effect almost staggers belief, and, upon merely a superficial view of the subject, Teluses its assent, to what I conceive to have been made sufficiently clear; and those doubts will be further strengthened when we recollect that the second edition of Dr. Hutton's work was published after a lapse of twenty-nine years, from the publication of the first; and at a time when the Commons of the United Kingdom had applied to him for his opinion upon the subject. This, together with his well-known abilities as a mathema tician, would have induced me also to think I was wrong, were I not convinced, both by theory and practice, that I am right. But we are now both before a discerning public, and it is for them to decide. Here, Mr. Editor, I shall close this subject, and likewise our correspondence, for the present, as I know of nothing more that appears to me very reprehen. sible, or likely to mislead my brother bridge-builders in their pursuit to attain knowledge in their profession. But, if tine and other circumstances will permit, I intend in another shape to furnish them with every information I am capable of affording them, both in theory and practice. And now, with thanks for the indulgence I have received from you, I conclude. JAMES PARRY, Bridge-builder. Bridgewater, Dec. 24, 1812. 1048 1 To the SI st asdf the Monthly Magazine. HA & I the too much gratified with al is 5o 1 ng account of the Honourg 59° 1' 18 Cavendish, in your Numbe in latitue last, to be inclined to fee 16.647". with it; but there is one sunsirent at that memoir which is cal. culated to make a wrong impression, and which a desire to do justice to my excellent friend, Dr. Hutton, induces me to correct. The assertion to which I advert is, that, at the top of column 2, page 421, where the deterinination of the mean density of the earth is ascribed to Dr. Maskelyne, and no mention whatever is made of Dr. Hutton, though he was undoubtedly the first person who ascertained that point. Had Dr. Maskelyne been living, I am persuaded that distinguished astronomer, and truly amiable man, would not have suffered so mistaken an assertion to poss without correction: but, as he has passed to other regions, and higher employments, and aş Dr. Hutton is, I believe, too much engaged in other concerns at present to enforce his own claims, perhaps you will indulge me with the insertion of the following hasty sketch of the leading proceedings relative to the matter in question. If the attraction of gravity be exerted, as Newton supposed, not only between the large bodies in the universe, but between the minutest particles, of which those bodies are constituted, it becomes exceedingly probable that the irregula 1813.] of the Mean Density of the Earth. rities in attraction, occasioned by protuberances and depressions on the surface of a planet, will in some cases be perceptible and appreciable: and hence it has been naturally inferred, that, where mountains are of a favourable magnitude, shape, and position, their attraction may actually be determined by experiment. Newton himself gave the first hint of such an attempt in his "System of the World," (Principia, lib. 3,) where he remarks, "that a mountain of an bemispherical figure, three miles high, and six broad, will not, by its attraction, draw the plumb-line two minutes out of the perpendicular." In truth, the effect of its attraction would not exceed 1' 18". The first actual attempt to determine the attraction of a mountain, was made by the French academicians, who measured three degrees of the meridian near Quito, in Peru, and who found Chimboraço, a very high mountare in that vicinity, to draw the plumb-lire 8" from the vertical, by its attractiontra This result, however, fell far short infewhat the ory might lead us to expect Sand, therefore, M. Bouquet expresconstis wish that the experiment might bree kinated in other places, and in forurable circumstances. Nearly forty years after, namely, in the year 1772, 3, and 4, the confirmation that such an experiment properly conducted, would furnish to the theory of the universal and mutual attraction of all matter, was the subject of frequent disquisition among the fellows of the Royal Society of London, at their meetings; and it was at length determined, that an extensive experiment should be undertaken under the superintendence of a person suitably qualified, both for the purpose of ascertaming the effect of the attraction of a hill, and, if possible, of inferring from thence, the mean density of the earth. The first business was to fix upon a hill favourably situated for the purpose. Dr. Maskelyne, in a paper published in the Phil. Transactions for 1775, recommended two places which he thought would be found very convenient; the one, on the confines of Lancashire and Yorkshire, where, within the compass of twenty miles, are four remarkable hills, Pendle-hill, Pennygant, Ingleborough, and Whernside; the other a valley, two miles broad, between the hills Helwellin and Skiddaw, in Cumberland. It was found, however, on closer examination, that neither of these localities possessed all the advantages that 7 might be wished; and a committee was in consequence appointed, among whom were Dr. Maskelyne and Dr. Hutton, "to consider of a proper hill on which to try the experiment, and to prepare every thing necessary for carrying the design into execution." Mr. Charles Mason, (well known for his astronomical tables,) and Mr. Smeaton, were among the most active in making the inquiry; and the latter, at length, informed the committee, that, in his opinion, Mount Schehallien, one of the Grampian bilis in the north of Scotland, possessed the desired properties in a very eminent degree; "being a very lofty and narrow ridge, very steep, extending a great length east and west, and very narrow from north to south." Mount Schehallien being thus deter mined upon, it became necessary to provide for the expense of the undertaking, and to appoint duly qualified persons to conduct it. As to the expense, it was defrayed out of a surplus remaining from the benefaction of his Majesty, that enabled Dr. Maskelyne to observe the transit of Venus in 1769; and no fitter person could be wished for to superintend the proceedings than Dr. Maskelyne himself, provided he could obtain leave of absence from the Royal Observatory, for a sufficient time to take all the nicer and more delicate observations. "This permission," says the Doctor, "his Majesty was graciously pleased to grant;" and, accordingly, the Astronomer Royal immediately prepared for the operations. He had two assistants, Mr. Reuben Burrow, who had previously been assistant astronomer at Greenwich; and Mr. William Menzies, a land-surveyor in Perthshire. These gentlemen measured all the lines, angles, elevations, sections, &c. which were judged necessary; and Dr. Maskelyne made a few of the nicer astronomical observations, as well as determined the deflection of the plummet from the vertical line, at convenient stations, on both sides of the hill. This business being accomplished, he returned to Greenwich, and prepared the general account of the measurements and observations, which is inserted in the Philosophical Transactions for 1775. From this memoir, in the Transactions, we learn that the sum of the deflections on both sides, occasioned by the attraction of Schehallien, was 11"-6. Dr. Maskelyne adds, "The attraction of the hill, computed in a rough manner, on supposition of its density being equal to |