332 Professor Dean’s method of illustrating the equation of time. 
never could be but two mean days in a year, and these would revolve 
through the year with the apsides. 
On Venus the greatest equation from the obliquity of its axis is 
2h. 24’ 16", the sun of course makes mean time four times in a year, 
and the eccentricity of its orbit produces but very small fluctuation of 
those times. On Mercury the greatest equation from the eccentricity 
is 14. 34’ 40", and unless the obliquity of its equator be more than 
double that of the earth, the mean and apparent time can never coin- 
cide but twice in one of their years. On Saturn the equation from 
the eccentricity is greater than from the obliquity, but less than double 
of it, of course it has sometimes two mean days, and sometimes four 
in a year; the other planets have only two. 
After twice delineating this draught certain mechanical facilities 
occurred which, should it be thought worth engraving, a workman 
might be glad to obtain. The curves are composed of segments of 
circles ; in the broad oval on the lower paper, one sign about each of 
the cardinal points is described on the centres p 2 = 1g respectively, 
and the intermediate segments of two sigs each, on the centres v, ¢,4,/, 
respectively. On the moveable paper the 3d »4th, 8th, 9th, 10th, and I1th 
signs were described with the radius of the mean circle, on a point distant 
from its centre by the greatest equation ; the Ist, 2d, 5th, 6th, 7th, and 
12th signs reckoning from A, with a radius about + 35 Of an ‘inch less, 
on centres to meet. It is scarcely necessary to mention, that the | par- 
allel lines expressing minutes are described on the same centres. with 
the corresponding segments of their principals. If the use of a thin 
transparent paper should be deemed inconvenient, it will be equally 
accurate, though not quite so ey: to draw parallels 0 on the outside 
of ae nde d 
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