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and west view, and n the north and south view; o is a granite pedestal, 6 feet long, 5. feet wide, 2 feet 10 inches deep, firmly resting upon a gravel foundation. The lower side of this block of granite is 7} feet below the floor. The sides of the transit room are supported on independent walls, separated from the base of the transit by a trench 3 feet wide, which has been filled up with tan to preserve the stones from being deranged by frost. Care has been taken not to allow the floor to press upon the piers where they enter it. By these precautions, the instrument is protected from the jar of footsteps in the observatory, and the disturbances of carriages. An excellent Transit-instrument, made by Troughton and Simms, feet long, was placed in adjustment in January, 1840, and a series of meridian observations, including moon-culminating stars, has been continued from that time, which, by their comparison, afford satisfactory evidence of the stability of the pillars on which the instrument rests. F F is the direction of the astronomical meridian, and intersects Bluehill, in Milton, west of its summit. A firm and substantial meridian mark has been erected on that hill, consisting of a tower of round and substantial masonry, thirteen feet in diameter at the base, seventeen feet high above the ground, and nine feet in diameter at the top. On this is placed a mark seven feet high, of the shape of the rhomb, with its larger axis perpendicular to the horizon. By this means the central vertical wire of the transit instrument is put in the meridian. It appears from Mr. Borden's State Survey, that the distance between the mark and the instrument is 58,520.5 feet. A brick house on the meridian line, about a mile from the transit instrument, affords a convenient, though less accurate reference, when the state of the atmosphere does not allow the distant mark to be distinctly seen.
A short abstract from the astronomical records of the Observatory is annexed, to show the state of the transit instrument, and the confidence that may be reposed in the accuracy of the time used in the magnetic observations. The equatorial intervals of the wires of the transit instrument, as deduced from a large number of observations, are thus :
33.94 From 2nd to mid-wire
16.88 From mid-wire to 4th
16.86 The following extract from the transit book includes all the standard stars, whose transits were observed from July 10th to July 25th, 1840. In a few cases, an observation has been rejected becaus one of the five wires was accidentally missed. The introduction of such stars would vitiate the general comparison.
15 11 a Libræ
-15 23 14 40 53.87 5.09 16 Ursæ Minoris +74 48 14 50 06.59 18.27 17 8 Libra
8 1815 7 16.05 27.18 18 a Serpentis + 6 56 15 35 15.03 26.36 19 B Scorpii -19 22 15 55 0.59 11.90 20 8 Ophiuchi 3 1716 4 49.95 61.18 21
Antares -26 4 16 18 28.67 40.10 122 li Scorpii +14 35 16 24 48.41 59.72 23 a Herculis +12 41 17 7 13.09 24.46
24 a Ophiuchi -27 52 17 25 22.45 33.80 25
Draconis +51 3117 51 45.11 56.62
* This difference has been corrected for the error of the clock. 7th column of the table, and may be seen on page 32.
This error is found from the
h. 43 14 3 Aquila +6 0119 46 16.58 30.66 " la Capricorni
-13 2 20 7 59.79 14.11 45 a Cygni +44 43 20 34 47.44 61.75 46 61 Cygni
+37 58 20 58 32.94 47.03 47 & Cygni + 29 35 21 4 56.42 10.86 48 “
a Cephei +61 55 21 13 34.03 48.06,149 " 3 Aquarii
6 16 21 21 56.99 11.42 Polaris 1st wire
0 40 3.80 2d
1 11 22.70
1 14.08'+ 0.08 1 14.32 0.23 1 14.31' + 0.03 1 14.09 + 0.24 1 14.44 0.35 1 14.03 + 0.42 1 14.43 0.39
mid " 4th " 5th 66
60 25 Polaris S.P.
a Cor. Bor.
The mid-wire bisecting the
mark on the Bluehill. +88 2713 0 47.50 +20 0114 6 54.82 24.25 1 29.43 +27 15 15 26 28.06 57.50 1 29.44 -0.00 + 6 56'15 34 56.60 26.14 1 29.54 0.09
Abundant materials are furnished, by the preceding catalogue of transits, for deducing the state of the instrument, the direction of the meridian, and the true sidereal time. By means of these observations the rate of the clock is found to be From the 10th to 15th July–1.16 From 15th to 25th July-1.41 s.
-1.43 -1.17 -1.12
-1.28 Mean from 10th to 25th July. There are three methods in common use for determining the azimuth of the transit instrument. One of them employs the successive intervals between the upper and lower passage of Polaris; another compares the transit of two circum-polar stars, whose right ascensions vary about 12 hours ; while the last depends upon the transits of high and low stars, including north and south stars. When the latter method is adopted, it is advisable to select stars whose difference of declination is at least equal to 40°. There are defects in all these methods. The first supposes the rate of the clock to be uniform during the 24 hours : this may not always be the case ; but on account of the slow motion of the star, a considerable error in time would make but a small difference in the azimuth of the instrument. The second process depends upon the clock for a much shorter time ; but it requires four observations, and the accidental error of these may exceed that produced in the first case by the clock. Lastly, the third mode requires only two observations, and depends on the clock only for a short time : but it supposes the tabular place of the star to be accurately known. A careful determination of the meridian must be based upon a combination of the three methods ; when this is done, the instrument is adjusted with great exactness. This reduction has been made out of the transits in the table, and the azimuth of the instrument, when on the meridian mark, calculated from 18 different sets of stars, selected according to the known conditions of the problem, gives, as a mean result, 04.072 west of south, being the azimuth of the south end of the
The three successive transits of Polaris, observed on the 14th and 15th July, give,
01.11 west of south. The final mean is 01.091 west of south. The octagon apartments to the west of the transit instrument contain a Gauss Magnetometer, by which changes of magnetic declination are observed. They are built of wood, with copper and zinc nails; the walls rest on wooden posts ; iron, stone, and all other substances known or suspected to exert magnetic influence, having been carefully excluded from every part of the building. F D is the direction of the astronomical meridian, and D C of the mean magnetic meridian. The three circles at D are the projections of so many wooden posts, which are bound firmly together at the top, and support the marble table on which a variation transit is placed. This instrument was made by Troughton and Simms, and is used in the magnetic observations. The larger interior circle represents the table; the chord at right angles to the magnetic meridian is the projection of the scale which is read in the observations : C is a Gauss Magnetometer, which is fitted up after the style of those at the Gottingen Observatory. The three circles at C are the feet of three posts, ten feet long, which unite at the top, eight feet above the floor, and give a stable point of suspension to the needle. The rectangle, enclosed within the feet, is the box which surrounds the magnet, and protects it from currents of air. Within the box may be seen this magnetized bar in the magnetic meridian, with a mirror firmly fixed to its south end to reflect the scale at D into the tube of the variation transit. The bar is suspended by a copper wire silvered, 0.011 of an inch in diameter, 5 feet 6 inches long, and, with its fixtures, weighs about 3 pounds.
From this arrangement it follows, that as the bar varies in its position, it must carry the mirror with it. The place of the mirror determines the mark on the scale which is reflected into the centre of the telescope ; so that as the reflecting surface moves, the marks of the scale that are successively seen are read off, and employed to ascertain the motions of the bar. The scale at Cambridge is so divided, that the angular motion of the needle is read directly from it without any reduction. This novel mode of observing changes of magnetic declination is a great refinement upon the old methods, and has given an accuracy to the determination of this magnetic element, which has hitherto been considered attainable only in astronomy. For a more minute description of the Gauss instruments, and the directions to be obeyed in observing with them, the reader is referred to Taylor's Scientific Memoirs, Parts V. and VI.* It is necessary to omit in this place any further details, except such as are required in order to understand the remarks which follow. As all the observations embraced in this paper were made with the
These instruments, directions, and the magnetic observatory, are fully described, with large folding plates, in vol. 3 of these Annals.--Edit.