PHYSICAL ASTRONOMY. 167 
the prism, in which the two images must be brought in contact; the index 
will then give the angular distance of the two objects at the station of the 
observer. In this instrument the colored glasses are wanting. 
92. Another kind of reflecting instrument formerly used in measuring 
angles of moderate value, is the Reflecting Sector (fig. 24), whose limb, DD, 
only contains somewhere from 10-15 degrees. The alidade carries the ver- 
nier HK, with the double tangent screw, FF, for fine adjustment; I and K are 
the mirrors, GH the telescope with the bent ocular, H, so that the observer 
at H looks downwards into the telescope. At the present time the instrument 
is no longer used, owing to the difficulty of rectifying it. Even the reflecting 
sextant is but rarely employed on land, theodolites having taken its place, being 
equally convenient to carry when of small size, and giving angles with much 
greater precision. At sea, however, the sextant retains full sway, as there 
no other observing instrument can supplant it. 
93. This is the appropriate place to refer to the triquetrum ( fig. 21), ar 
ancient instrument, supposed to have been invented by Ptolemy, for deter- 
mining altitudes and amplitudes of the heavenly bodies. It consisted of a 
staff, A, placed vertically by the assistance of a plummet, D. Attached to 
this staff were two others, B and C, movable on hinges, and thus capable of 
forming various triangles with the first. On one of them, namely on B, 
were placed the sight vanes, a and 6. The construction and use of the 
triquetrum (so called from its triangular shape) depended upon correct 
geometrical principles, although, as is very evident, observations made with 
it could be of but very superficial character 
The Sun-Dial; the Gnomon. 
94. Sun-dials are instruments by means of which the true solar time 
can be determined, when the sun is above the horizon and not obscured by 
clouds. Before the invention of wheel clocks, they formed the only means 
for an accurate determination of time. Gnomonics, a special department of 
applied mathematics, teaches the mode of constructing sun-dials on any 
plane or curved surface. Even the Egyptians were acquainted with the sun- 
dial; at least, Josephus expressly asserts that the obelisks served for astrono- 
mical observations; and Augustus caused an Egyptian obelisk to be erected 
in Rome for the same purpose. The Jews had them 732 B.c.; and as to 
their existence among the Greeks, they are to be found in the choragic 
monument of Andronicus Cyrrhestes in Athens. Papirius Cursor constructed 
the first sun-dial at Rome 290 B.c. Portable sun-dials were invented by 
Pope Sylvester in the tenth century. 
Sun-dials consist generally of a face of proper form—the dial surface— 
upon which is an hour ring; on this latter, the shadow of a style or gnomon 
indicates the hours. 
There are various constructions of dials, depending upon the position and 
character of the dial face. The simplest form, and the one most usually 
employed, is the egwinoctial or equatorial dial, whose plane is parallel to the 
167 
