TIME OBSERVATIONS. 175 
which we may mark I, and so on. The distance of the Sun above the Equator 
will evidently make no difference in the lateral direction of the shadow. In 
practice however, we do not want such a cylinder; all that is necessary is a 
projection called a "style," parallel to the axis of the earth, like the axis of the 
cylinder, and a dial; the inclination above a level here will be 39° 06' at the 
north end of the style. 
The principle of both clocks and watches is that a number of wheels, locked 
together by cogs, are forced to turn around, and are prevented doing so too 
quickly. The force which gives the 'motion may be either a weight or spring; 
the force which arrests the too rapid motion may proceed from a pendulum, 
which at every swing locks the wheels, or from some equivalent arrangement. 
In both clocks and watches we mark the flow of time by seconds, such that 
sixty make a minute, sixty of which make an hour, twenty-four of which make a 
day. Those who are not astronomers are quite satisfied with this, and a day is a 
word with a certain meaning. The astronomer, however, is compelled to qualify 
it— to put some other word before it — or it means very little to him, because the 
term day may mean either the return of a particular meridian to the same star or 
to the Sun again. The term as it is commonly used, means neither the one nor 
the other, because long ago, when it was found that in consequence of the motion 
of the earth not being uniform in its orbit round the Sun, the days, as measured 
by the Sun, were not equal in length, astronomers suggested, with a view of 
establishing a convenient and uniform measure of time for civil purposes, that a 
day should be an average for all the days in the year. So our common day is 
not measured by the true Sun, as a sun dial measures it, but by what is called the 
mean, or average, Su?i. 
For a long time after watches and clocks were made with some degree of 
accuracy, it was attempted to make them keep time with the sun dial, and for 
this purpose they were regulated at short intervals, and ignorant persons would 
blame the clock-maker for making an imperfect machine that would not keep 
time with the sun-dial. 
Having said so much of solar days, both apparent and mean, let us next 
consider the starting points of these reckonings. We have first the apparent 
solar day reckoned from the instant the true Sun crosses the meridian through 
about twenty-four hours, till it crosses it again. Second — The mean solar day 
reckoned by the mean Sun in the same manner. Both these days are used by 
astronomers. Third — The civil day commences at the preceding midnight, is 
reckoned thiough twelve mean hours only to noon, then recommences, and is 
reckoned through another twelve hours to midnight. The civil reckoning is 
always twelve hours in advance of astronomicaL Sidereal time is reckoned from 
the first point of Aries, and is the only time that can be taken without several 
corrections. When the mean Sun occupies Xho. first point of Aries, which it does 
at the vernal equinox, the time by the mean and sidereal clocks will be the same. 
This happens at no other time of the year. 
A sidereal clock represents the rotation of the earth as referred to the stars, 
