Of course, excursions were planned by railroad companies and others to points on the line of the shadow, the usual directions for observing were extensively circulated, and the eclipse was made the catch-word for many advertisements whose substance had no connection with it. We are afraid that many persons may have lost the most beautiful features of the scene by a too persistent use of smoked glass, which of course was not necessary during or even near the time of the total obscuration.
The weather for some days previous was not very promising—not on account of too much rain, but owing to the absence of it; and every evening the sun set in a bank of haze, which each day seemed to increase, and no storm occurred to clear the air of the burden accumulated by the drought. This was particularly unpromising for the photographers, who needed really clear air for good work; the times of beginning and ending, to which, formerly, great importance was attached, could probably have been observed nearly or quite as well through haze, or even thin cloud.
We have just implied that less consequence is now attached to the time observations than was formerly the case; this is due to the great perfection which the lunar and solar theories have now attained, which is such that the prediction of the positions of the sun and moon, and even of the beginning and ending of an eclipse, can be made with greater accuracy, perhaps, than almost any one observer could note them. Still, by combination of all the results, some slight corrections to the tables now used may perhaps be deduced, and on the present occasion this portion of the work was not disregarded, but provided for with all the appliances of modern science.
The recording of time is now usually made by the electric method, which may be here described briefly, though many are probably familiar with it. The principle is the following, subject to various modifications in the particular form of apparatus: A line is described by a pen made to move uniformly over the paper by means of clock-work. That this line may be indefinitely prolonged without retracing, it is usual to make it a spiral round a horizontal cylinder, which revolves, say, once a minute, while the marking-pen (otherwise stationary) moves slowly from one end of the cylinder to the other, perhaps requiring several hours for the complete passage.
The pen making this line is held in its place by the action of an electro-magnet pulling against a spring; the circuit through this magnet is broken every second by the escapement of a clock or chronometer; the magnet then for an instant ceases to act, and the spring pulls the pen aside, making a break in the line at regular intervals corresponding to every second of time. The same interruption of the circuit can also be made by an observer provided with a key like those used by telegraph operators, and the time of his observation thus registered on the chronograph, as the instrument is called. For identification of the clock-mark preceding his observation, mechanical arrangements can easily be devised, by which the first second in each minute shall be omitted, the circuit not being broken; so that it will be known what second of every minute each mark corresponds to; and the fraction of the second elapsed from this clock-mark to his own can easily be estimated by the eye, or measured more carefully. The reading of the record is, of course, facilitated by having the cylinder revolve once a minute, so that all the clock-marks answering to any particular second (as the twenty-third, for example, of each minute) will come in the same horizontal row; and the marks are not made on the cylinder itself, but on a sheet of paper fastened round it, which can be detached when filled.
Instruments of this character were used at Shelbyville, and also at the border stations near the edge of the path of the shadow, but inside of it, one of which was at Falmouth, about thirty miles south of Cincinnati, the other at Oakland, near the Mammoth Cave. The observations of time were especially important at these places, since, as will readily be seen, the length of time required for a circular or elliptical shadow to pass a point near its edge will vary very rapidly for a slight change in the size of the shadow, or a slight shifting of its path toward or from the point selected. Even rough observations, merely of the duration of the eclipse, made at two such stations on opposite sides of the central line, suffice to determine with great accuracy the dimensions and precise track of the shadow, and thus give the elements of the moon's motion.
We have just spoken of the shadow as being elliptical; this was of course the case, the sun being quite low at the time, so that the round cone of darkness, technically known as the umbra, was cut very obliquely at the earth's surface. To realize the amount of this ellipticity or distortion, one would only need to hold some spherical body so as to cast a shadow on the ground about an hour and a half before sunset. The elongation was also continually increasing as the sun sunk toward the horizon, and its direction changed as the sun at the same time changed its direction or bearing, the longer axis of the ellipse always pointing toward the sun. This axis was, in Kentucky, about three hundred miles long; the shorter ninety; and this elliptical patch of darkness was moving in a course some thirty degrees south of east, or about twenty-three degrees south of its own longer diameter; its speed was about seventy-five miles a minute, or more than the average on the whole track, as before stated, and it required rather less than three minutes to pass any given point on the central line; this was consequently the duration of the totality; and short enough it certainly was, for the amount of work which was to be done by the observers.
For the stations on or near the central line, it was important to obtain the absolute times of the contacts, and for this purpose transits were observed, to get the error and rate of the chronometer, for some time before and after the eclipse. The border observations locate the path on which the shadow travels, and determine its breadth; but to obtain the position of the shadow on this path at any fixed time, the true times of its arrival and departure at fixed points must be observed. But on the border no such preparations were necessary, only the interval being required; and a simple pendulum, without clock-work, was set up for this purpose, which broke the circuit at each second, and thus left its record, serving to count the number of seconds and the fraction between the beginning and end of the totality, which were observed and similarly recorded by means of a break-circuit key. This pendulum was so arranged as to break the circuit on the main telegraph line, and thus to be heard, and record its beats at a number of stations in different towns; but the main circuit did not itself mark upon the registers used by the observers, but mechanically (by means of what is called a relay magnet) broke short circuits set up at their stations, which could also be broken in another place by their own keys, without, of course, interfering with the main circuit itself; so that every observer could receive the pendulum beats upon his own record, without receiving those made by observers at other stations.
On Thursday afternoon, the 5th of August, some showers occurred, but not sufficient, according to ordinary experience, to have much effect in clearing the atmosphere; and on Friday morning the sky became overcast with mackerel clouds of a most unpromising character. All the preparations were, however, hopefully continued, and the photographer, Mr. Whipple, of Boston, took on that day some very successful views of Shelbyville, of the college buildings, and of the party of observers. The principal station had been established in the grounds of the college, the instruments being protected by a large tent; close by was the Coast Survey station, where the chronographs just described for recording time, as well as a transit instrument for observing it, had been placed.
Friday evening was cloudy at Shelbyville, but without rain, and the chance seemed to be gradually diminishing of any thing like a good observation of the eclipse.