330 Professor Dean’s method of illustrating the equation of time. 
represent the line of apsides. From this circle, was set off at the dis- 
tance of every 10° the equation of the sun’s centre converted into time, . 
on the same scale with the other, but the position of the points is re- 
versed, the negative equation being on the outside and the positive on © 
the inside, and the paper cut through those points. By this aversion 
the sum of the two equations when they are alike, and their difference 
when unlike, is easily discerned, whatever may be the position of the 
line of apsides. When the space expressing the equation contains the 
circle of mean time, the two causes conspire to produce the effect, 
but when this space lies wholly without or within the circle of mean 
time, the result is the difference of the two causes. Where these 
curves intersect each other, it is evident that the positive and negative 
causes, being equal, cancel each other, and the apparent time coincides 
with the mean. 
Turning this paper again and again, backward and forward, and 
comparing with the tables, has led me to some conclusions, which bare 
reflection on the causes may have produced in some minds, though it 
never suggested themto me. J cannot say they are very valuable, but 
they may serve to amuse a speculating moment, and possibly suggest 
relations on other subjects, leading to important conclusions. The 
equation by the obliquity of the equator being the greatest, there must 
be always four days in each year, that is one belonging to each of the © 
cardinal points of the ecliptic, on which the mean and apparent time 
coincides. © These days I beg leave to call mean days, and they never 
can be more than 27 days from that in which the sun passes those 
points. _ Each moves through its 53 or 54 days forward in about 
12716 years, and then returns in about 6920 years. The eq inoc ial 
mean days advance into the succeeding months when the sun on them 
is nearest to its apogee, and retrogade when it is nearest its perigee 
On the contrary, the solstitial mean days reeede wher the sun on thes 
