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POPULAR SCIENCE REVIEW. 
place within the course of a single year. If there are only 
two eclipse-months, these may be of any of the above classes, 
but not both of class 1. If both are of class 3, there are only 
two eclipses, and both are solar and either total or annular. 
Thus we see the reason of the statement commonly made 
without assigned cause in popular works on astronomy, that 
there are never less than two or more than seven eclipses, and 
that if there are only two, both are solar. To this we may add 
the rules that, if there are seven, four are solar ; and that, if 
there are two, the moon is obscured four times in the terrestrial 
penumbra. 
It is also evident, that the most important eclipses are likely 
to take place when there is a single solar eclipse during the 
passage of the critical period. This happens twice in the year 
1868. One of these eclipses took place on February 23. It 
was annular, and visible so near to us as the northern p^rts of 
France. The other will take place on August 17, and will be a 
very remarkable eclipse. It will be visible over the regions 
indicated in Plate XXX. 
We have seen why a single solar eclipse (during the eclipse- 
month) is likely to be a noteworthy phenomenon. Let us next 
consider what other circumstances atfect the magnitude of an 
eclipse. 
The earth moves around the sun in an elliptic orbit, her 
greatest and least distances from the sun being respectively as 
31 to 30. The moon, also, moves round the earth in an elliptic 
orbit, her greatest and least distances being as 10 to 9. Thus 
the apparent diameters both of the sun and moon are variable ; 
the diameter of the sun varying between the values 32' 36'' *4 
and 31'31"-8, that of the moon between the values 33'31"*1 
and 29'21"*9. Thus at the epoch of central eclipse the sun 
may be wholly obliterated or a ring of light may be left un- 
hidden. The extreme cases are — (1) when the sun’s diameter 
has its greatest value and the moon’s its least, in which case 
there will remain a ring of light 1'37"*2 wide; and (2) when 
the sun’s diameter has its least and the moon’s its greatest 
value, in which case the moon’s disc overlaps the sun’s by 
5 9" *6 all round, and the sun continues, therefore, for several 
minutes wholly obliterated. 
Fig. 3 shows how far the cone of total shadow reaches in the 
former case, c is the vertex of the cone; the lines acu' and 
a'cu would, if produced, touch the lunar and solar discs. In 
this figure the distance ac bears its just proportion to the 
earth’s dimensions, but the angles aca', ucu' are greatly ex- 
aggerated. The lines pp, p'p' bound the penumbra: and the 
cone aca' contains all points from which the sun would appear 
to be annularly eclipsed. 
