842 TRANSIT TRANSIT CIRCLE tween the two durations thus noted could be determined at once ; whereas in Delisle's meth- od each observer had to determine the absolute time of a single phenomenon, and a compari- son between their results could only be effected satisfactorily if these results could fee referred to some common standard time of reference, as Greenwich or Paris time. But in the ac- tual application of both methods another diffi- culty obtruded itself into notice. It was found that the moment when Venus was in internal contact, either at ingress or egress, could not be determined, as Halley had hoped, within a single second, or indeed within several seconds. Accordingly doubt had long rested on the de- termination of the sun's distance obtained from the observations made in 1761 and 1769. In fact, from the first, the results were found to be widely discordant according to the manner in which the observations were interpreted. The values of the sun's distance deduced from the transit of 1761 ranged from 77,846,000 m. to 96,163,000 m. ; those deduced from the tran- sit of 1769, though not ranging quite so widely, yet differed by more than 4,000,000 m., the greatest being as before 96,163,000 m., the least 92,049,650 m. Strangely enough, all this was forgotten when (after Encke had published his result from the combination of both series of observations, viz., 95,265,000 m.) a long pe- riod had elapsed during which the text books and ephemerides had published the same value for this important element. Accordingly, much surprise was expressed when other meth- ods of observation showed that this value so long received was too great by three or four million miles, the true value appearing to be nearer 92,000,000 m. Although this surprise was by no means justified by the facts of the case, yet it was natural that much attention should be attracted to the transits of 1874 (Dec. 8) and 1882. Accordingly great preparations were made for the observation of the earlier transit, the United States in particular taking a distinguished part in the work. It has been es- timated that nearly $1,000,000 must have been expended on the various expeditions. Stations were occupied in Siberia, China, Japan, the Ha- waiian islands, northern India, Persia, Turkis- tan, and Egypt in the northern hemisphere, and at a number of islands in the Indian and South- ern oceans, from Kerguelen on the east to Chat- ham island and New Caledonia on the west, Australia, Tasmania, and New Zealand being also occupied in force. Various success attend- ed the observers, but on the whole the results obtained were excellent. Delisle's method and Halley's, the heliometric method, and photog- raphy were applied at many of the most im- portant stations ; and though bad weather pre- vailed at other stations, the object of the expe- ditions was achieved. So far as can be judged at present, the sun's distance indicated by these observations is about 92,000,000 m. The next transit of Venus will occur Dec, 6, 1882, and is looked forward to with great interest for verifying these determinations. The transits of Mercury are much more frequent than those of Venus, in consequence of the former planet being nearer the sun, and having thus a nar- rower orbit and a shorter year ; but they are not available for the determination of the solar parallax. The transit of stars is employed in the determination of longitude. (See LONGI- TUDE.) The precise relative situation of the heavenly bodies in respect to their right as- cension is determined by comparing their ex- act times of transit. For the means by which these times are ascertained see TRANSIT CIECLE. TRMSIT, Engineer's. See THEODOLITE. TRMSIT CIRCLE, an astronomical instru- ment for determining the absolute positions of the heavenly bodies. As these positions are given by two independent elements, the right ascension and declination, corresponding to geographical longitude and latitude, so this in- strument is a combination of two independent Transit Circle. constructions, each giving its share to the name of the whole, and each furnishing its corre- sponding element by independent and yet simul- taneous observation. The transit circle now forms an essential part of the equipment of every well constituted observatory. The two constructions which have here combined their powers are the transit instrument and the me- ridian or vertical circle. The former consists of a telescope whose tube is composed of two slightly conical portions firmly secured at their bases to opposite sides of a hollow central cube, from two other opposite sides of which proceed also equal cones of more massive make, gener- ally indeed cast in the same piece with the cube, and forming an axis at right angles with the telescope. At or near the extremities of this axis are two perfectly cylindrical, highly finished pivots of hardened steel, correspond- ing in position to sockets resting upon stone columns which, based firmly in the ground,
