The celestial meridian. A surveyor's transit. 80 are nine parallel wires, and one at right angles to them. Eight are arranged symmetrically with respect to the middle wire. They come from the spider's loom; woe to the luckless wight who accidentally touches them, or blows upon them! They are in the focus of the telescope, close to the observer's eye, inside of the tube. If the telescope be directed to the heavens on a clear night, star after star will pass through the field of view, marching across one vertical wire after another, moving parallel to the horizontal wire. When a star is just crossing the middle wire, it is on the celestial meridian of the place of observation, if the instrument is in perfect adjustment. Let us stop a moment and think out the reason why a star is on the meridian when it is on this middle wire. FIG. 80.-THE SPIDER-WEBS. Consider a surveyor's transit which he carries about and sets up on its tripod whenever he wishes to make any measurements. In it there are two cross wires, one horizontal, the other vertical. he wishes to sight at the If top of a church spire he moves his telescope until the tip of the spire appears to lie on the intersection of the cross wires. At that instant a straight line drawn from the top of the steeple through the center of the objectglass of the telescope strikes the point where the two wires cross each other. This line is called the sight-line of the telescope. Returning to the meridian circle we see that its sight line, which is a line drawn from the center of the object- upon the middle wire the chimney is due south of the instrument. Passing by the house we prolong the sightline to the celestial sphere, which it strikes at the south point of the horizon. We gently take hold of the telescope and pull the eye-end down; as it turns on the horizontal axis the object-glass rises, and the sight-line traces a line on the celestial sphere. Farther and farther upward the line is traced on the sky till it reaches the zenith. As we go on, the circle which we have been tracing runs down from the zenith to the north point of the horizon. The telescope is now horizontal, and pointing northward. We continue revolving the telescope in the same direction; the eyepiece rises and the object-glass falls, while the sight-line is cutting into the earth's surface, tracing upon it the terrestrial merid FIG. 81.-THE SPIRE ON THE CROSS WIRES. *The large glass at the upper end of the tube. The meridian circle again. The telescope is revolved. Mechanical perfection sought. Perfection impossible. ian of the place of observation. When the telescope finally reaches its original horizontal southward-pointing position, the sight-line has traced the celestial meridian on the sky, and the terrestrial on the earth. If the celestial meridian were visible as a fine gold thread lying on the celestial sphere, and one tried to look at it with the meridian circle, it would be concealed from view, being behind the central spider-web. Therefore, at the instant when any star appears to be crossing the central spider-web, it is on the meridian. Thus far we have considered the meridian circle as an ideally perfect instrument. True it is that the mechanician has exhausted the resources of his art when he has made a first-class meridian circle. He has striven to make the pivots at the ends of the axis of the same size and exactly round. The telescope has been set at right angles to this; the object-glass and spider-webs have been inserted with the utmost care. Upon the graduations of the silver circle weeks of the most painstaking labor, coupled with the most scrupulous care, have been lavished. The microscopes with which the circle is read have been constructed with an eye to perfection. The interior of the glass level-tube, which is to test the horizontality of the axis, has been ground to the proper curvature, and fastened to its supporting frame in such a way that changes of temperature will not cause the tube to be pinched or sprung. The mason has endeavored to set the supporting piers so solidly that nothing short of a miniature earthquake will disturb their positions. The astronomer views the finished work with the admiration which every one must have for any piece of mechanism which represents the utmost of human skill. But the instrument, which is to the eye of the body a thing of beauty, is to the mind a mass of imperfections. Flexure. Errors of graduation. Level errors. Movements of the ground. The pivots on which the instrument revolves are of unequal sizes, and neither of them is round. For this reason alone the sight-line, instead of tracing a perfect circle on the sky, traces a gently waving line. The axis, which is apparently amply able to support the light telescope, bends a trifle under its weight; perchance one half of it bends more than the other half. The telescope tube flexes under the weights of the object-glass and of the eye-end. Changes of temperature and other causes alter the position of the objectglass in its cell, and change the direction of the sightline, which passes through its center. The exquisite silver circle will cost the astronomer many a month of arduous toil. For if he assumes that one of the graduations is exactly in the right place, almost all of the remaining 4,319 are so far out of their true positions that he must determine their errors. As we have before stated, he wishes to read as small a quantity as Tʊʊʊ of an inch, and most of the circledivisions are in error as much as ʊʊʊ of an inch; some of them are over zʊʊʊ of an inch out. The little micrometers on the microscopes cannot do their small duties with sufficient precision. The inner surface of the level tube, which has been ground so smooth, is embellished here and there by a miniature mountain, which arrests the free movement of the level bubble. The solid foundation on which the instrument has been set is continually in motion, shifting the positions of the piers by small amounts. Earthquakes are only the big brothers of the many small disturbances of the earth's crust which are noticed by astronomers alone. The observer with a meridian circle has therefore a A difficult task. difficult task; he must manipulate the instrument with exceeding care, and must study many of its errors from |