THE CHRONODEIK. THE FOLLOWING DESCRIPTION OF THIS INSTRUMENT IS TAKEN FROM THE SCIENCE OBSERVER," VOL. III., No. 3. "The instrument designated by the above title, is one which was devised to furnish a convenient means for finding the time within a second. It will be found to possess very great advantages in cheapness and simplicity over any instrument now in use, and is perfectly adapted to the wants of astronomical amateurs, watchmakers, surveyors, and exploring parties. This instrument, the principle of which is entirely new, consists of a swinging bar, suspended, at its upper end, on a pivot, in such a way as to permit the bar to assume freely a vertical position without any torsional revolution. To the bar is affixed a small telescope, the object-glass near the bottom, the eye-lens at the top of the bar. Below the object-glass a frame is fixed to the bar, carrying a plane mirror swinging on a horizontal axis and provided with a clamp for fixing the mirror at any desired inclination. Below this is a metal bob. The whole instrument thus forms a pendulum, which is suspended inside a large tube, at the base of which are approximate leveling screws. A smaller tube at the top carries a dark-glass directly over the eye-lens of the enclosed telescope, without touching it. The cut shows the general external appearance of the instrument. The mirror, with the milled heads by which it is revolved and clamped, is seen through the apertures near the bottom of the enclosing tube. From the above description of the construction it is evident that, if the instrument is turned so that the mirror is toward the sun, and the mirror is revolved on its horizontal axis until the rays from the sun are reflected directly up the telescope tube, an eye looking into the instrument from the top will see an image of the sun in the field of view. As the sun rises or falls in the heavens this image will rise or fall in the field of view, and its passage may be observed over a horizontal bar or wire placed in the common focus of the eye and object-glasses and so adjusted in direction that it represents a section of a small circle in the heavens paralled to the horizon. This brief explanation will indicate the mode of using the instrument in getting the time by equal altitudes of the sun. For this purpose it possesses the most surprising facility. In cheapness and simplicity it has, of course, very greatly the advantage over the sextant-the use of which, with the artificial horizon, requires considerable skill, and involves much trouble and inconvenience—as well as over the small, portable Watchmaker's Transit, which requires careful mounting and adjustment, and even more skill and trouble than the sextant, in practical use. The instruments which have so far been constructed are very compact, and are portable in the coat pocket. They have object-glasses of six inches focus, the magnifying power being seven or eight diameters. The whole instrument is about one and three fourths inches in diameter, and stands about nine inches high. With instruments of this size and construction, the probable error of a single determination of the clock error with one instrument may be definitely stated to be within one second of time. Indeed, the actual results of an extended series of experiments with several instruments shows the probable error of a determination with one instrument to be somewhat less than this, or eighttenths of a second. This degree of accuracy is probably sufficient for the purposes of most of those who will be likely to use the instrument, although it could easily be enhanced. DIRECTIONS FOR MAKING AND REDUCING OBSERVATIONS WITH THE CHRONODEIK. For Observations on Morning and Afternoon of the Same Day. Set the instrument at any convenient time in the morning, not very near noon, so that the long aperture in front of the plane mirror faces the sun. Level by the foot-screws, so that the pendulum swings freely. Loosen the clamp-screws, (right hand milled head), and, looking into the telescope, turn the mirror by the left hand milled head until the sun appears in the field of view. Adjust and clamp the mirror so that the sun, when the pendulum hangs free and still, will be a little below the bar or wire in the field. Turn the instrument laterally, so that the sun is a little to the right of the centre of the field, and will in rising diagonally, by the diurnal motion, touch the wire near the centre. Observe the time, by watch, when the first limb or edge of the sun touches the wire, and also when this edge is completely across the wire. In general, this interval will be from twelve seconds to eighteen seconds. Record the mean of these times. About three minutes after the 2d limb of the sun will pass the wire. Observe and record this in the same way as the first limb. Set the instrument carefully aside until a corresponding time after noon. Then set the instrument in the sun, level, and, without disturbing the mirror clamp, observe and record the passage of both limbs of the sun, as in the morning observation. Reduction.—Take the mean of the times of the passage of the sun's first and second limb at the morning observation; also, the mean of the two times at the afternoon observation; then take the mean of these two means. The Chronometer time of local apparent noon is then found by the Equation of Equal Altitudes. For Observations on Afternoon of one Day and on Morning of next Day. In this case the only differences to be noted in the above directions are the following: At the first or afternoon observation the mirror is to be adjusted and clamped, so that the sun will be a little above the wire. It may not be out of place to add the following precautions:- Never observe through a window pane; even the best plate glass will distort the sun's image and vitiate the result. Arrange to observe the contacts of the sun with the wire as near the middle of the field as possible. Never disturb the mirror clamp between the two observations, as upon the fixity of this adjustment depends the precision of the result. If the sun is partially obscured by cloud, the observation can be often made by removing the cap containing the dark glass. The most rapid motion in altitude occurs, of course, when the sun is near the prime vertical, or directly east or west. It is scarcely necessary to say that in carrying the instrument about, between observations, it should be held in a slightly inclined position, so that the pendulum, resting against the side of the tube, cannot swing about and jar the mirror adjustment." The Chronodeik appears to be a very ingenious illustration, or application, of well known laws of Optics and Gravitation. The thread placed in the common focus of the eye and object glasses should be parallel to the direction of the circle of equal altitude at its intersection with the vertical circle passing through the body observed. Practically, I presume, this is effected by fixing the thread parallel to the horizontal axis of the mirror; and this adjustment should be sufficient, provided the centre of the thread can be closely estimated. I think the instrument would be improved by using a second thread, perpendicular to and bisecting the first. The Chronodeik is restricted essentially to the approximate determination of time. The sextant may be used for the same purpose, and it seems unnecessary to argue that it is the best instrument in general use for the closely approximate determination of angular measurements. The description of the Chronodeik says that "the actual results of an extended series of experiments with several instruments shows the probable error of a determination with one instrument to be about eight tenths of a second. This degree of accuracy is probably sufficient for the purposes of most of those who will be likely to use the instrument, although it could easily be enhanced." When the sextant is used the same formula for reduction is employed. Professor Chauvenet remarks, "The chief source of error is in the observation itself. The most practised observers with the sextant cannot depend on the noted time of a single contact within 03 5, and hence the intervals between the successive chronometer times (which, if observations could be perfectly taken, would be sensibly equal) may differ 2. But the greatest probable error of the chronometer time of sun's or star's transit, from the mean of six such observations on each side of the meridian, is found to be not more than 0 23, provided the rate of the chronometer between the observations is uniform." (Chauvenet's Lunar Distances and Equal Altitudes, page 72.) Using either instrument, errors due to changes in refraction may be almost wholly removed by computation. With the Chronodeik the observations are restricted to the transit over a single thread of the two limbs of the sun, while with the sextant the noted times of contact may be numerous. Stars may be employed with the latter instrument, but not with the former in its present construction. The Chronodeik may be improved by enlarging it, and substituting a reticle for the single thread, with means of illuminating the reticle, but in its present form it is apparently inferior to the sextant. It is doubtful that the uses of the instrument would justify the improvements and increased expense. P. F. HARRINGTON. |