OBSERVATORY AT WEST POINT. 193 
THE EQUATORIAL. 
The only fixed instrument in place, is in the central tower. It is an equatorial by 
Mr. Thomas Grubb, of Dublin, which is believed to possess some merit. Its object is to 
measure by direct observation the apparent declination and hour angle of any heavenly 
body. By means of the latter and the sidereal time of observation, the apparent 
right ascension, which is equal to their sum, becomes known. ‘The instrument consists, 
essentially, of two axes of motion at right angles to each other, two graduated circles, one 
attached to each axis at right angles to its length, and a supporting frame. When the 
instrument is adjusted for use, one axis is parallel to that of the earth, and the other to 
the plane of the equator, the former being called the polar and the latter the declination 
axis. 
In principle, Mr. Grubb’s instrument resembles that of Fraunhofer. In the arrange- 
ment of its parts, it is, however, different, and very simple. Fig. S represents a per- 
spective view of this instrument taken from a point to its north-east, the telescope being 
. in the meridian. Fig. T’ exhibits a meridian section, the declination axis being in this 
plane, and the telescope directed to the pole. ‘The polar axis, (1) Fig. J, is about forty 
inches long between the bearings, is of a conical shape, terminating towards the vertex 
in a well turned steel pivot about three-eighths of an inch in diameter, and at its base in 
an accurately turned cylinder, one inch in length, and about eight inches in diameter. 
It is hollow for a distance of more than three-fourths of its length from the larger end, 
and is mounted in the centre of two acute hollow conic frustrums (2), (3), which are 
united to each other by screw-bolts, as represented at (4), so as to have a common axis. 
The diameter of the largest base is twenty-two inches, and that of the smallest ten and 
a half. Connected with a flanch (9), at the largest base, are two substantial pieces of 
wrought iron (5), united in a crucial form at right angles to each other, and perforated at 
the place of union, which is in the common axis of the frustrums. This perforation 
receives a cylindrical box of iron with a large bearing head (6), and this latter a solid 
cylindrical piece of Brazilian metal, terminating at one end in a bearing head (7), in 
which is a small cavity for the reception of the lower end of the polar axis, and at the 
other in a screw for a nut, which keeps it in place. This latter cylindrical piece is 
smaller than the opening through the box (6), and may be moved laterally by means of 
four small screws (8), which enter the box at right angles to each other. By this device, 
the more delicate adjustments of the polar axis are made. The collar, or upper bearing 
surface of the polar axis, rests upon two pieces of Brazilian metal (10), Fig. C. which are 
let into the inner surface of the smaller conic frustrum, at its smaller base. These pieces 
are about three quarters by one inch on the rubbing surface, and of a proper shape to 
receive the cylindrical form of the axis. They are 90° apart, each being 45° from the 
meridian; and are relieved from a great portion of the pressure, by a friction roller (11). 
The polar axis has, at the larger end, a square flanch, projecting outward so as to measure 
eight and a half inches on a side; the edge of this flanch is seen at (15), Fig. S, and to 
its lower surface, the hour circle (13) is attached. The hour circle is of brass, and in 
shape resembles, when in place, an inverted shallow cylindrical cup, with a circular hole 
in its base just large enough to receive the polar axis. ‘The graduation is on a band of 
VOL. IX.—52 
