270 
POPULAK SCIENCE KEYIEW. 
be as small as possible. We have seen that the least value this 
element can have is 31' 31" *8, the greatest 32' 3f)"*4. On 
August 17 the sun’s diameter will be 31' 41"*0, or 9"*2 greater 
than the least and 55"*4 less than the greatest value. Of this 
element then we can merely say that it is favourable. But the 
apparent magnitude of the moon is a more important element. 
It should of course be as large as possible. We have seen that 
it varies between the values 29' 21"*9 and 33' 31"*1. On 
August 17, it will be no less than 33' 28"*6 ; only 2"*5 less than 
the greatest value this element can have. 
Then, lastly, as respects the latitude of the regions traversed 
by the eclipse. This should be such that the sun should rise 
nearly to the zenith of the place at which the eclipse is central 
at noon. In the present instance the sun is only 2^° from the 
zenith of the spot where this happens (longitude east from 
Grreenwich 102° 50'*6, north latitude 11° 35'7). In this region 
the total eclipse lasts 6m. 50s. 
The last-named peculiarity of this great eclipse is illustrated 
in Plate XXX., which exhibits the path of the moon’s shadow 
across the earth’s disc.* This plate requires a little explanation. 
It serves to illustrate what is termed the projection of solar 
eclipses : in this method the earth and moon are supposed to be 
seen from the sun. In such a case the transit of the moon 
across the earth would, of course, determine the occurrence of 
an eclipse — since if the moon hides any part of the earth from 
the sun, the sun must be hidden (wholly or partially) from that 
region of the earth ; in other words, the region must be in the 
shadow of the moon. 
Now, there are three motions to be considered in dealing with 
Plate XXX. First the earth, as seen from the sun, is moving 
bodily from right to left at the rate of upwards of 65,000 miles 
per hour. This motion is indicated by the long transverse arrow. 
Then the earth is rotating upon her axis in such a manner that 
regions visible on the disc appear to be moving from left to 
right. This motion is indicated by the small arrow placed on the 
equator. Lastly, the moon apparently traverses the earth’s disc 
from left to right, or in the same direction as that of the former 
motion— but at a greater rate. In fact, the apparent motion of 
the moon (as supposed to be seen from the sun) during the 
eclipse of August 17, is about twice as great as that of the equa- 
torial parts of the earth. This motion is indicated by the small 
arrow beside the path of the black shadow. 
* The presentation and slope of the earth’s axis are deduced, with a slight 
variation, from Plate IX. of my Sun-views of the Earth.” The continents 
and oceans are, in fact, presented in almost exactly the same way in fig. II., 
as in the first figure of the above-named Plate. 
