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* TRANS-ISTHMIAN CANAL. length of canal navigation from Caledonia Bay to the tide lock about 50 miles. The result of all these examinations showed tliat there was no probability of the existence of any practicable canal location between Panama and the mouth of the Atrato River, except by the adoption of a tunnel line, and there seemed to be no reason for further field work except in the interest of geograpliv. See Report of Nicaragua Canal Commission, " 1897-99 (Baltimore, 1899) and the bibliography under Nicaragua Canal and Panama Canal, which articles should be read in this connection. TRANSIT INSTRUMENT (Lat. tmnsitus, passage over). An astronomical instrument consisting principally of a telescope fixed to a horizontal east-and-west axis, and revolving in the plane of the meridian. It is employed, as its name denotes, in the observation of the meridian transits of the heavenly bodies, i.e. in noting the exact instants when they reach the meridian of the observatory. The axis, which is the most important part of the instrument, and thus demands the utmost care in its con- struction, consists of a hollow sphere or cube, to opposite sides of which are tightly fastened the bases of two cones in whose apices the pivots are screwed; the sphere or cube is pieced for the admission of the telescope, which is firmly fas- tened at right angles to the axis. One of the pivots is hollowed so that a stream of light can be directed from a lantern half way along the interior of the axis, and through an aperture in the side, into the telescope tube, where, being received by a small reflector, set at 45° to the axis and telescope tube, it is directed to the eye- piece, rendering visible a series of fine spider- threads stretched across the field of view. The pivots must be carefully turned to a perfectly cj'lindrical form, and fitted into the instrument, so that they are accurately in line. One ex- tremity of the axis carries one and sometimes two small graduated circles, each supplied with index, clamping screws, and vernier; these circles are capable of indicating angular measures to within 1' or 2'. There are three adjustments Deeessary before a transit instrument can be used: the axis must be horizontal; the sight line of the telescope, as marked by the spider-threads, must be at right angles to the axis of motion ; and the latter must be placed so as to point accurately east and west. On the perfection of the first two of these adjustments depends whether the telescope sweeps o'er a great circle of the sphere, anil the third is necessary to in- sure that this great circle shall be the meridian of the place of observation. These adjustments can never be made quite perfect, and the usual mode is to investigate the amount of error in each, and allow for it by a calculated correc- tion applied to the observed time of the star's crossing the meridian. To note accurately this observed time by the astronomical clock is, as we have said, the essential part of a transit ob- servation. It is best done by means of a chrono- graph (q.v.), an instrument with which an auto- matic record of the observed time can be made by means of an electric signal sent out directly from the observer's hand. The transit used by engineers will be found described and illustrated omder Engineering Instruments. 417 TRANSMISSION OF POWER. TRANSIT OF VENUS. The passing of Venus across the sun's disk, seen when the planet is exactly in line with the sun and the earth. The two planets, Mercury and Venus, whose orbits lie between the sun and the earth's orbit, can at times pass between us and the sun. We can then see them in the telescope, appearing as black dots projected on the luminous solar surface. If the exact instant of time be observed when the planet first touches the solar disk, and again when it finally leaves it, we can use the observa- tions to calculate the solar parallax and the dis- tance from the earth to the sun. Transits of Venus are especially well suited for this purpose, and they have been very elaborately observed in the past, especially in the years 1874 and 1882. Unfortunately, experience has proved that it is impossible to observe the required instants of time with sufiicient precision ; and other methods of measuring the sun's distance are now pre- ferred by astronomers. (See Parallax; Eros.) The next transit of Venus will occur June 8, 2004. Transits of Mercury will happen on the following dates in the twentieth century: November 12, 1907. November 6. 1914. May 7, 1924. November 8, 1927. May 10, 1937. November 12, 1940. November 13, 1953. November 6, 1900. May 9, 1970. November 9, 1973. November 12, I98(i. November 14, 1999. It will be noticed that transits of Mercury occur much more frequently than those of Venus ; but, as we have said, they are not well adapted for solar parallax observations, and are indeed of quite minor importance astronomically. TRANSKEI (trans-ki') TERRITORY. A dependency of Cape Colony, between the Great Kei River and Tembuland. Area, 2552 square miles; population, in 1891, 153,563, the region thus being the most densely populated part of Cape Colony. The territory is administered by a chief magistrate and is subject to the 'Native Territories Penal Code.' TRANS'LEITHA'NIA. A name applied to the eastern division of the Austro-Hungarian monarch}' on account of its position east of the Leitha River, which forms a part of the boundary line between the two divisions of the monarchy. See Cisleithania. TRANSMIGRATION. See Metempsycho- sis. TRANSMISSION OF PO'WER by Elec- tricity. The electrical transmission of energy in considerable quantities from the point of generation to one or more distant points of delivery. This excludes telegraphy and telephony (qq.v.), where energy is transmitted over a wire, but in small quantities. Energy is transmitted in order to utilize a cheap source of energy supply, and the total cost of generation and transmission to the point of use must be less than would be the cost of generation at the point of use, otherwise such transmission would not be profitable. Therefore, transmission plants are usually supplied by water power or by cheap coal, and the electricity is derived from generators. (See Dynamo- Electric Machinery.) The cost of the trans- mission depends upon the distance and upon the permissible electrical pressure, or voltage, of the transmission line. The weight of copper re- quired to transmit any amount of energy for any