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HA RD WICK& S SCIENCE- G OS SIP. 



knowledge of their contours, and, by the aid of the 

 micrometer, their actual dimensions can be deter- 

 mined. The crater-mountains are, as Webb observes, 

 " the grand peculiarities of the moon ; " many 

 thousands of these have been delineated, all present- 

 ing more or less the same features — a terraced ring- 

 wall with a central hill. From sunrise on the moon 

 to sunset, the shadows are perpetually changing, 

 except, of course, when the moon is full and nothing 

 prevents the reflected sunlight reaching us from all 

 parts of the surface. It is for this reason that the 

 moon is not nearly so interesting an object when full 

 as when it presents a crescent or gibbous shape. 

 These crater-mountains are on such a vast scale, 

 relatively to the size of the moon, that the few crater- 

 plains which are known to exist on the earth sink 

 into insignificance in comparison ; the diameter of 

 Copernicus, for instance, is fifty-six miles, and it has 

 a peak rising to a height of 13,500 feet. The most 

 noticeable features of the full and nearly full moon 

 are the systems of bright streaks which emanate from 

 a few of the craters, especially Tycho. These curious 

 rays are not caused by their being elevated above, or 

 depressed below, the surrounding surface ; nor has 

 any satisfactory explanation of their presence been 

 hitherto brought forward. One of the most remark- 

 able formations is the Straight Wall, well described 

 by its name, extending for sixty miles, and having a 

 uniform height of 1000 feet. There is so much 

 variety in the lunar surface that the study of even 

 one detail may well occupy a considerable time ; if 

 the observer is a good and rapid draughtsman, he 

 will act wisely in making drawings of what he sees. 

 At any rate, he should have a note-book, and make 

 records therein at the time of observation. Never 

 should he trust to the next day's recollections, for 

 doubts will certainly arise. It is only by making, 

 preserving, and afterwards comparing, careful records 

 that we can hope to detect any changes in the 

 heavenly bodies. 



Hitherto all our observations have been made with 

 the equatorial and its appliances, but the practical 

 astronomer needs other instruments. It is essential 

 that the time of every observation should be accu- 

 rately noted ; he must, therefore, have the means of 

 finding the true time, and for this purpose a transit 

 instrument is required. This consists of a telescope 

 very carefully mounted so that it can be moved only 

 in the plane of the circle which passes through the 

 north and south points. The object-glass need not 

 be large ; in the case of portable instruments the 

 aperture is seldom more than two inches, but in 

 large observatories this size is often much exceeded ; 

 the Greenwich transit circle, for instance, has a tele- 

 scope with an object-glass eight inches in diameter. 

 The value of the instrument does not depend on its 

 light-gathering quality, but upon the rigidity of its 

 mounting and the accuracy of its single movement. 

 Attached to one side of the mounting is a graduated 



circle, and by means of verniers moving with the 

 telescope (or vice versa) the inclination of the latter 

 to the horizon is known. Having found from the 

 almanack the altitude of a bright star, it is evident 

 that, if the telescope is adjusted to that angle, the star 

 will be seen in the field of view when crossing the 

 meridian. The interval between two consecutive 

 passages of the same star over the meridian is a 

 sidereal day, whereas the length of the solar day is 

 determined by two consecutive meridian passages of 

 the sun, and differs slightly from the former, the 

 value of which is 23 h. 56 m. 4*091 s. Clocks are 

 made to show sidereal time, and one such is always 

 within view of the observer when stationed at the 

 transit instrument. But the size of the field of view 

 depends on the power of the eye-piece used, and the 

 accurate determination of its centre, that is, of the 

 position of the meridian, is well-nigh impossible. 

 Evidently some contrivance must be adopted, and 

 we again call the services of the spider to our aid. 

 A positive eye-piece being used, at its focus is placed 

 an arrangement of wires (web), generally five in 

 number, one horizontal and passing through the 

 centre of the field of view, bisected at right angles 

 by a second, the others being placed at equal dis- 

 tances on either side of, and parallel to, the central 

 vertical wire. By noting the times at which a star, 

 moving along the horizontal wire, appears to be 

 bisected by each of the vertical ones, its passage 

 over the middle wire, or, in other words, over the 

 meridian, is easily determined. When the first 

 point of Aries is on the meridian a sidereal clock 

 shows oh. om. os. Knowing the sidereal time and 

 the right ascension of the star whose transit we wish 

 to observe, it is easy to find the mean time at which 

 that phenomenon will take place, and thereby, if 

 necessary, we can correct our mean time clock. It 

 is usual to select suitable objects from a list of clock- 

 stars, whose places have been determined with a 

 considerable amount of accuracy. As the wires of 

 the eye-piece are invisible on a dark night, provision 

 is made for their proper illumination. The axis of 

 the transit instrument consists of two truncated cones, 

 one of which is hollow in order to permit the light 

 from a suitably placed lamp to impinge on a mirror 

 placed in the main tube at an angle of 45 , by which 

 it is reflected to the wires. A somewhat similar 

 arrangement is generally adapted to the equatorial. 

 Before "taking a transit" one must notice the time 

 by the clock, and then count the ticks of its seconds 

 pendulum, estimating the intervals to the tenth part 

 of a second. This is distinguished as the "eye and. 

 ear method," and is being gradually superseded by 

 the' use of electrically controlled chronographs. A 

 barrel, on which may be placed a sheet of paper 

 divided into small squares, is kept revolving by 

 clockwork at a certain rate. Insulated wires con- 

 veying a current of electricity are led from the 

 chronograph to the eye-end of the transit instrument 



