IOM 



ASTRONOMY. 



[THE Mr HAL CIRCLE. 



of tho Greenwich J.iadrant by a direct comparison of the 

 renulU of the two instrument*. Since that time, circular 

 instruments have been very generally employed. The 

 improved method of graduation, first introduced by 

 ,-hton, in 1800, and the previous invention of the 

 micromotor- microscope by Itamsden, have allowed artist* 

 to give to this instrument the greatest degree of per- 

 fection. Tho first large instrument of this class appears 

 to have been the Greenwich mural circle, which was com- 

 pleted by Trough ton iu 1811, and which, in several 

 detail*, may almost be considered as a new instrument. 



! 111 



Some years previously, the celebrated Ramsden made a 

 circle of five feet in diameter, with which Piazzi framed 

 his great catalogue. The idea, however, is an old one, 

 ncr, although better known as the inventor of 

 the transit instrument, used a meridian circle for several 

 years, similar in construction to the modern form. In 

 his letter to Leibnitz, December 15th, 1700, he mentions 

 that he preferred an entire circle of four feet in diameter 

 to a quadrant of eight feet radius. Of his observations 

 with the meridian circle, extending from December, 1704, 

 to December, 1710, only three days remain, tho others 

 having been destroyed by tho fire which ^occurred at 

 Hafnia, October 2nd, 1728. These three days' observa- 

 tions, from October 20th to October 23rd, 1700, have 

 been carefully reduced by Dr. Galle, and contained in his 

 Inaugural LHuertation, printed at Berlin, in 1845. The 

 diameter of this meridian circle and the length of the 

 axis was five feet and a quarter ; the length of the tele- 

 scope fixed to tho axis was five feet. The circle was 

 "read off" by two microscopes placed at an interval 

 of 10. 



Tho mural circle consist* of tho graduated circle (Fig. 

 211) fixed in the mi-rid inn, and supported by a long axis, 

 on which it revolves. This axis passes through a block 

 of stone, or solid masonry, the weight of the instrument 

 being taken off the pivots by means of counterpoises 

 placed at the back of the pier. The circle is generally 

 graduated on it* rim from to 300', and also into 

 mailer subdivisions of 5' of arc. 



The instrument is furnished with a telescope, B B, 

 firmly fixed to tho axis, and perpendicular to its plane, 

 with which it revolves. Tho divisions aro road by 

 micrfiniptcr-micrfxicopefi, F, generally six in number, 

 placed at equal distances around it* circumforon< 

 object being to eliminate, by readings of opposite 

 diameter*, error* arising from eccentricity, flexure of tln< 

 axis, and expansion. Tho SIM of the instrument, at tho 



principal observatories, varies from four to eight f. 

 diamotor. The Greenwich circles were six feet ; tl. 

 Cambridge is eight feet in diameter. The number of 

 microscopes made use of at Oxford and Kdinlun 

 four ; at Greenwich and Cambridge six are constantly 

 read off. It was formerly thought necessary to have two 

 instruments at Greenwich for determining a star's place 

 namely, a transit instrument and Troughton's mural 

 circle ; but a larger transit circle has been constr. 

 for the Observatory, under the direction of tl 

 Astronomer Royal, which has been in use sii 



Tlse, telescope of this iimtrument lias an aperture 



.cs, and a focal length of 

 : ilf : a length of axis bt'twu-n the cxtrc- 

 1" tho pivots of six feet, the diam< 

 each pivot being six inches. Tho circle is six feet 

 in diameter, and of cast iron. 



The axis of the circle is made horizontal by the 

 aid of a plumb-lino suspended iu frout of the 

 circle, and viewed l>yt\\o microscopes one near 

 the top, the other near the bottom of the 

 or, as this instrument is supposed to bo used in 

 conjunction with a transit instrument, the axis 

 may be made horizontal by moving tho adjn^tin^ 

 screw, go as to make a zenith star pass the middle 

 ire at the instant the star is passing the middle 

 wire of the transit instrument. 



The adjustments of the meridional position of 

 the mural circle are made by the observations 

 of certain stars hi conjunction with the transit 

 instrument. The reading microscopes should 

 accurately describe 300", or five revolutions be- 

 tween each graduated space on tho limb of tho 

 circle. This can be approximated to by proper 

 ad justing- screws, which regulate the distance of 

 tho microscope from the limb ; but, notwithstand- 

 ing the accuracy with which the adjustment may 

 V, it is found that unequal temperature will 

 alter these numbers' considerably. The error 

 arising from this circumstance is called tho 

 "correction for runs." It is the practice in 

 observatories to determine tlio amount of this 

 quantity frequently which applies to a microscope 

 reading of 5' and to form corrections proportional for 

 6ther circle readings. The following is an example 

 (Cambridge Observations, May 3rd, 1834) : 



The telescope is furnished with a system of vertical 

 wires similar to the transit instrument, and a fixed hori- 

 zontal wire placed at right angles to the others. This 

 wire is generally adjusted so that an equatorial star will 

 continue bisected during its transit ; but it is sometimes 

 left with a small inclination, which can be easily observed 

 by comparison with the movable horizontal wire. Several 

 stars being bisected at the first and seventh vertical wires, 

 tho effect of inclination can be readily determined for the 

 whole, by comparison with the micrometer of the mov- 

 able horizontal wire. To determine the value of one 

 revolution of this micrometer, any distinct terrestrial 

 object may be bisected at on position of tho micrometer, 

 and tho circle carefully "read off." The same mark is 

 again to be bisected at another position of tho micrometer, 

 an 1 the circle is again to be "read off." When tho two 

 circle readings have been properly reduced, and tho runs of 

 tho microscope having been applied, their difference will 

 be a value of circle reading, in minutes and seconds, equal 

 to a certain number of revolutions of the nn 

 This operation being repeated at different micrometer 

 readings, a current value of one revolution of the tele- 

 scope micrometer can bo obtained. Tho more distant 

 and distinct the terrestrial object selected is, the better. 



A very convenient transit circle has been erected for 



