PHYSICAL ASTRONOMY. 169 
perpendicular to the meridian, and passing through the poles; the index is 
here parallel to the equator. There are also cylindrical dials where the 
surface is a cylinder; and annular, where the hour circle is marked on the 
inside of a ring. The rays of the sun falling through a hole in a hoop upon 
this circle, determine the hours. The portable dials are principally hori- 
zontal, and must be set up by means of a compass. 
It remains to remark, in conclusion, that as the sun-dials indicate only 
the true, and watches the mean time alone, the two can only agree exactly 
twice in the year. 
96. The Gnomon was a contrivance of the ancients, ta determine the 
altitude of a luminous body above the horizon, by the shadow cast by a 
vertical style upon a horizontal plane. Anaximander made use of the 
gnomon to determine approximately the obliquity of the ecliptic at 24°; and 
after that, Pythias and Hipparchus calculated the solstices and altitudes of 
the sun. In all probability the obelisks of the Egyptians were nothing else 
than such gnomons, by means of which they obtained the culmination of the 
sun. and consequently the true noon, for the purpose of regulating their 
water clocks. An improvement of this apparatus is presented by the 
Thread Gnomon, in which the solar rays are received on a vertical wall 
perpendicular to the plane of the meridian, and the precise position of the 
meridian plane, passing through the centre of a circular aperture in the 
wall, indicated by a depending thread. It will be readily understood that 
the meridian lines required for each gnomon must be previously determined 
with the greatest possible degree of accuracy. 
The Wheel Clock. 
97. By the word clock, without further qualification, is meant every 
machine which, by means of the perfectly uniform motion of wheelwork, is 
intended to divide mean, solar, or sidereal time, into a certain number of 
equal parts; the minuteness and accuracy of these latter depending on the 
more or less complete elaboration of the component parts of the apparatus. 
It is an ascertained fact that the first clocks were moved by weights. 
Galileo and Huyghens first applied the pendulum to regulate the motion of 
the clock by its regular oscillations. In the sixteenth century a spiral spring 
was used as a motive power instead of the weights, and the pendulum was 
replaced by the balance wheel. By means of these two substitutions, it 
became possible to reduce the mechanism of the clock within so small a 
compass, as to render it sufficiently portable for pocket use ; and although 
the honor may be contested against him, Peter Hele of Nurnberg is to be 
considered as the inventor of watches. 
98. This is not the place to go into a minute Behcitpiled of the mechanism 
of a clock; it must be remarked, however, that the motive power, whether 
bent spring or weight, acts upon a wheel with a certain number of teeth, 
and that by a proper arrangement of wheels and pinions, the indices are 
moved in such a manner that the one (the minute hand) makes twelve 
169 
