visible it was followed by a faint flame-shaped tail, corresponding to the yellowish portion of the discharge.

Having thus proved the existence of an isolated discharge, at once the first and most brilliant act in the explosion, a method was contrived for measuring its duration, or at all events for setting a limit to it on at least one side. Ordinary micrometric methods, conducted on the naked spark or on a strip of paper illuminated by it, were out of the question, and would have served no purpose except to lead the observer into error, so I contrived the following plan, which is moderately easy of application and safe.

A small piece of cardboard was ruled with two black lines; their distance apart was 0067 inches, and they were separated by a space exactly equal to the width of a single line. A small dividing engine was used for their production; they were tested with a microscope. These lines were illuminated by the spark, and their image examined by an eye-piece magnifying five diameters. The breadth of the image of a single line on the ground glass was 011 inches, that is, each line subtended an angle of 2′ 24′′, reckoning from the mirror. With a velocity of 223 per second the mirror is able to move the reflected image through 2' 24" in the 00000024 of a second. If now this first discharge had actually lasted this minute portion of time, it is evident that the motion of the mirror would just have carried the image of one of the black lines forward, so that at the end of this infinitesimal period, it would have occupied the space where just before the white line had been traced: hence owing to the retention of impressions by the retina the white central line would have been obliterated, and in place of three lines, a gray band would have been seen. On the other hand if the duration had been only or of the above mentioned interval, the white line not having been much encroached on, would still have remained visible. The correctness of the above reasoning can be experimentally proved, by means of a revolving disc of card board with a single slit cut in it, lines being drawn on its white side, and viewed by reflection with a mirror through the slit, the blackened side of the disc being turned toward the eye.

To facilitate matters, three sets of these lines were drawn on the small cardboard at considerable distances apart to prevent confusion, and while illuminated by the electric spark were examined with increasing velocities. With low rates they were quite unchanged in appearance, with a velocity as high as 102 per second, a duration of the first discharge of 00000052 seconds would just have obliterated them; they were however perfectly distinct, though more faintly traced: the rate of ro

tation was then by degrees carried up to 223 per second, when the lines were still distinctly visible, though of course with less contrast between the white and black, than was the case with low velocities or a stationary mirror.

This experiment proves then, that the duration of the light accompanying the first discharge is considerably less than •00000024 of a second, probably less than half this period, or less than one ten millionth of a second of time. To make the observations required some patience, as it was necessary to use an eye-piece, the ground glass being replaced by a plain polished plate, and it was seldom that the image of the lines fell exactly in that portion of the field to which the attention was directed. To obviate this difficulty, I afterward covered a piece of cardboard half an inch broad with alternate white and black lines of this character, their real diameter was 0075 in., that on the plate of glass being 0125 in. With these I repeated the above mentioned set of experiments with rapidity, obtaining with ease the same result, as some of the lines were almost certain to be in the right part of the field of view. It is thus shown, that the first act of the electric explosion lasted through an interval of time so short as to be immeasurable with the means then at my disposal; it is not impossible that it may still be reached by the use of finer lines, and a lens of greater focal length.

From the foregoing then it appears that if a jar having a metallic coating of about 100 square inches, be connected as above described with an induction coil, its discharge will be effected by a considerable number of acts, of which the first is by far the most intense. Farther, the metallic particles heated up by the first discharge to a white heat, almost instantly assume a lower temperature, marked by a corresponding change from white to brownish yellow, and as their temperature continues to fall the tint changes in the case of brass electrodes to green, in that of platinum to a gray or violet-gray. A spectroscopic examination of these isolated tints would be interesting but not without difficulty. These observations farther demonstrated the fact, that two ten millionths of a second is an interval of time quite sufficient for the production of distinct vision.

When the light from the spark is received directly on a plane revolving mirror, and viewed by the eye as in Wheatstone's original experiment, only the white unanalyzed portion of the spark is ordinarily visible; at least in repeating the experiment a few times, it is all that I saw. Its form is of course

that of the spark itself. In all probability this is also the case in a jar charged by frictional electricity, and may serve to explain the great discordance between the results obtained by Wheatstone and Feddersen, the method used by the former furnishing only a view of the first act, the eye being too much dazzled to perceive those that are subsequent and of far less intensity.

ART. XVI.-The Star-Mapper; by HENRY M. PARKHURST.

EARLY in the year 1856, I commenced the construction of a star-mapper, designed to extend the principle of the Chronograph, which maps right ascension only, to an instrument mapping both right ascension and declination. On the 7th of October, 1856, having completed with my own hands the necessary apparatus, I took my first map.

In my first apparatus a concave lens was moved in declination, perpendicular to the axis of the eye-piece, the telescope being fixed in the meridian and the stars as they crossed the field of view were thus optically brought in succession to the crossing of two fixed illuminated wires. A few levers converted this motion into a horizontal motion of the marking point or star-key, at right angles with the motion of a roll of telegraph paper one inch wide carried by a clock movement. Striking the star-key, at the instant of the transit at the center of the field, mapped the star in its proper position. In order to take a map several degrees in width, I omitted the concave lens and moved the telescope itself in declination, with a slightly varied system of levers.

So far there was no difficulty. But a star-mapper which would only map in the meridian would have been to me valueless; and I reconstructed and rearranged very many times the system of levers by which I could convert the visible motion at any desired parallactic angle into a horizontal motion of the star-key. My experience in the fall of 1856 convinced me that my apparatus had two radical defects.

1. The motion of the telescope to obtain the declination was troublesome, even for a 3-foot telescope, owing to the necessity of overcoming at every star the inertia and momentum of the whole telescope, and the consequent inaccuracy; and it would have been impossible with my means to mount my 8foot telescope, which I procured the following year, with such accuracy as to make this method of mapping possible. And further, when the telescope is clamped we have twice the ini

tial motion with the same field, and the error from any mechanical inaccuracy is therefore proportionally less.

2. Several of my early plans involved the motion of the star to a fixed point in the field. I abandoned this, substituting the motion of the point to the star, for two reasons.

(1.) It required a continual motion of the eye-piece, (or telescope,) to bring within the field the stars to be mapped; for while mapping a northern star, a southern star which ought to be mapped would be outside of the field.

(2.) It brought within the field twice as many stars as I desired to map, with no line of demarcation between them.

In 1860, thinking that I had worked with my pine mappers and narrow paper long enough, the instrument being readily adjustible and convenient for mapping in any desired position, I procured my first permanent star-mapper, chiefly of brass, adapted to map 90' in declination upon the scale of the Chacornac charts. In 1863, desiring to make charts upon twice that scale, I had another and larger mapper constructed, varying only in unimportant details. This latter instrument I will proceed to describe.

A large diagonal prism placed in the meridian inverts the image in declination, bringing the north point to the top of the field, the stars moving toward the left. Then if I wish to map in the eastern sky, I turn the prism to the left upon the axis of the telescope, and turn the field-piece with its attachments the same angle upon its own axis, also to the left, the angle being such as will be most convenient to the eye, the plane of the field making an angle of about 60° with the horizontal plane.

1.

Figure 1 represents the plane of the field of view. In the center of the field of view is represented the star-point, by which the stars are to be successively occultated, there being a projection upon the left, easily seen in the absence of illumination, to mark the exact point of occultation. This star-point describes an arc, the center of motion being represented to the

right, outside of the field of view, and being firmly supported by the field-piece. The line connecting the center of motion with the center of the field is parallel to the apparent motion of the stars. Into the field piece is fitted a positive eye-piece of suitable magnifying power.

The arc described by the star-point is exactly equal to that described by the star-key; and the levers connecting the two are so adjusted that moving the star-key across the width of

the map shall exactly carry the star-point across the field of view. Unless it is desired to record the right ascension of the stars by the chronograph simultaneously with the mapping, it is better thus to map in corresponding curves than to provide means, as I did in my early apparatus, for moving both the star-key and star-point in straight lines.

The star-point receives its motion from a star-point-bar, which turns upon the same center of motion. A circular disk two inches in diameter, carrying the star-point, is clamped to a similar disk on the left end of the star-point-bar, as represented, at such an angle that the bar shall be horizontal when the star-point is in the center of the field.

2.

E

The star-mapper must next be adjusted at a convenient height and distance, so that the arm of the star-key, when in the center of the map, shall be parallel to the star-point-bar. Fig. 2 represents the connecting levers, as seen from the left. K represents a cross section of the further end of the arm of the starkey, playing, as thus seen, horizontally to the right and left. It is connected by a rod with a universal joint at each end, to the right-angled lever pivoted at A. A perpendicular rod adjustible in length communicates the motion to a horizontal lever, whose center C may be adjusted in height. Turning on the same center C is a bar D, which communicates the motion through a rod E, of variable length, with universal points, to the star-point-bar B, previously adjusted, a cross section of the further end of which is represented in the figure. The angle of the bar D to the horizontal bar may be adjusted by a clamp upon the connecting bar. By the adjustment of this angle, and of the position of the mapper, the connecting rod E must be brought into the plane of the motion of the bars B and D, and at right angles with them.

All these adjustments having been properly made, when the star-key is at the center of the map each connecting rod will be in the plane of the motion of each bar with which it is connected, and at right angles therewith; and these rods being of suitable length, moving the star-key will not vary these relative positions so far as to produce any important want of uniformity of motion of the star-point. The adjustment of the rate of motion of the star-point may be most conveniently made, either by varying the effective length of the bar B, or of the bar D, or both.