night to night, because they continually change in inexplicable ways. His occupation is largely an unrelenting

chase after errors, which must be determined and taken

into account.

A chronograph is considered an indispensable part of A chronograph. the instrumental equipment of an observatory. It is used, as its name indicates, for noting time. At any instant when an observer wants to note the time he touches a telegraph key, and the chronograph records the time. The large cylinder shown in Fig. 83 revolves once a minute. If the pen-carriage stood still the pen would

The pen

The pen carriage slides.

draw the same circle over and over again on the paper
which is wrapped around the cylinder. But the mech-
anism is so arranged that the pen-carriage slides slowly
from one end of the cylinder to the other.
therefore traces upon the paper a long spiral line, like a
screw-thread. When a telegraph operator presses his
telegraph key the sounder by his side clicks. If a pen
were suitably attached to the sounder, the pen would
make a mark on paper. In a similar fashion a notch is
made in the line which the pen draws on the chrono-
graph sheet, whenever an observer presses the key.

The record of the clock.

The clock is equipped with a little device which acts like an automatic telegraph key, causing the pen on the chronograph to make a notch whenever the clock ticks, with the exception of the fifty-ninth second of each minute, for which there is no record on the chronograph. The omission of this second is a matter of convenience, to identify the beginning of each minute. If the ob

server notices the time when one of the clock notches

58

The time noted.

The micrometer.

FIG. 84.-A PORTION OF A CHRONOGRAph Sheet.

was made, he can easily tell what the clock read when any other notch was made.

When he sees a star cross a spider-web in the meridian circle and touches his key, a notch is made which usually comes between two of the clock notches. If it is between the notches for ghr. 28min. 3sec. and 9hr. 28min. 4sec., the fractional part of a second is estimated from the relative distances between the notches. One of the notches shown in Fig. 84 was evidently made at 9hr. 28min. 3.4sec. It is much easier for an observer to touch a telegraph key at the proper instant than to estimate the required time by listening to the ticks of a clock, while his eye is occupied at the eyepiece of the telescope.

The micrometer is used on all kinds of astronomical instruments wherever small distances are to be measured accurately. It aids in reading the silver circle on a meridian circle; the diameters of planets, the heights of mountains on the moon, and the distances of the stars are all measured by its help. It is beyond our present province to explain how the minute fractions of an inch which a micrometer measures are transmuted into miles in the celestial spaces, by the alchemy of the mathema

micrometer.

tician's art. But we may at least see what the great micrometer which is screwed on at the eye-end of the Agreat Lick telescope looks like, and get a little insight into the method of its manipulation. Looking through the eyepiece, we shall not be confronted by a forest of spiderwebs, as in the meridian circle. It will suffice if there are but two fixed wires crossing

each other at a right

angle, just as in the surveyor's transit. Besides the fixed.

wires there must be one movable one, which is parallel to one of the fixed wires. The concealed frame, which holds the movable wire, is driven by a fine screw, the A fine screw. large head of which is visible at one end of the box. This head is graduated so that thousandths of a revolution of the screw can be read. If the screw has fifty threads to the inch, an entire revolution of it will cause the movable spider-web to move of an inch. One hundredth of a

revolution will cause a motion of 50% of an inch.

A planet's diameter.

If the diameter of a planet is to be measured, the movable spider-web is driven, by turning the screw, until the image of the planet in the field of view is neatly

Distance between stars.

uring, which the astronomer does at his leisure. When the apparent distance between two stars is to be measured, the micrometer box, containing the spider-webs, is turned till the two parallel webs stand perpendicular to a line

Fig. 86.--Measurement of a Planet's DIAMETER. joining the stars. At the completion of the measure the spider-webs are bisecting the images of the stars, as shown in Fig. 87.

In reducing observations made with the micrometer

no such tantalizing chain of errors is encountered as with the meridian circle. If the micrometer screw were Errors. of even pitch throughout its length, so that each revolution of it advanced the spider-web just of an inch, all would be well. When the irregularities in the screwpitch, which are always very small, have been determined, the battle is won.

If, however, one of the spider-webs is accidentally broken, the insertion of a new one demands a little skill. The astronomer cannot sweep down one of the cobwebs in the observatory to get a suitable wire. House-spiders are too effeminate; their webs are not sufficiently tough, and are covered with dust. A big field-spider, which successfully copes with an unwary grasshopper, binding his struggling victim by weaving a shroud about him, produces a web that is elastic and tenacious. The cocoon, in which are stored hundreds of yards of gauzy fiber, is captured. By the exercise of a little dexterity a piece of web three or four inches long is pulled out and placed under a magnifying glass. It proves to be too thick, and is rejected. Another piece is examined; curious little knots are strung along it. The next piece, when held up to the light, is too transparent. Soon a fine, smooth, opaque bit of web is discovered; it is submerged in a basin of water and stretched out, while soaking, so that it becomes finer yet. Inside A broken of the micrometer are two fine grooves. One end of the web is laid in its groove, with the aid of a magnifying glass, and a drop of shellac is dropped upon it; the

FIG. 87.-BISECTION
BY SPIDER-WEBS.

spider-web.