ound south of the
observatory bordering on the Arago Boulevard. A large movable
structure serves for covering the external part of the instrument.
This structure rests on rails, upon which it slides toward the south
when it is desired to make observations. It will be seen from the
figure how the principal axis of the instrument rests upon the two
masonry pillars, one of which is 49 ft. and the other 13 ft. in
height.

[Illustration: FIG 1.--THE GREAT EQUATORIAL OF THE PARIS OBSERVATORY.]

The total cost of the pavilion, rolling structure, and instrument
(including the two objectives) will amount to about $80,000 after the
new equatorial has been provided with the scientific apparatus that
necessarily have to accompany it for the various and numerous
applications to which the use of it will give rise.

[Illustration: FIG 2.--OCULAR OF THE GREAT EQUATORIAL.]

Fig. 2 shows us the room in the observatory in which the astronomer,
seated in his chair, is completely protected against the inclemencies
of the weather. Here, with his eye applied to the ocular, he can,
without changing position (owing to all the handles that act at his
will upon the many transmissions necessary for the maneuvering),
direct his instrument unaided toward every point of the heavens with
wonderful sureness and precision. The observer has before him on the
same plane two divided circles, one of which gives the right
ascensions and the other the declinations, and which he consults at
each observation for the exact orientation of the equatorial.

[Illustration: FIG. 3.--DIAGRAM SHOWING THE COURSE OF A LUMINOUS RAY
IN THE GREAT EQUATORIAL.]

All the readings are done by the aid of electric lamps of very small
dimensions, supplied by accumulators, and which are lighted at will.
Each of these lamps is of one candle power; two of them are designed
for the reading of the two circles of right ascension and of
declination; a third serves for the reading of the position circle of
the micrometer; two others are employed for the reading of the drums
fixed upon the micrometric screws; four others serve for rendering the
spider threads of the reticule brilliant upon a black ground; and
still another serves for illuminating the field of the instrument
where the same threads remain black upon a luminous ground. The
currents that supply these lamps are brought over two different
circuits, in which are interposed rheostats that permit of graduating
the intensity