ASTRONOMY. 145 



observations of that kind should be made as pos- 

 sible. 



When several such observations, made under two 

 different meridians, are compared, the mean may 

 furnish a tolerably exact determination of the dif- 

 ference of longitude. 



The comparison of the beginning or end, with cal- 

 culations previously made, may also serve for find- 

 ing the longitude, and may be useful for that pur- 

 pose at sea. 



Solar Eclipses. 



147. The length of the moon's shadow is less 

 than that of the earth, in the same ratio that 

 the diameter of the moon is less than the dia- 

 meter of the earth ; that is, in the ratio of 1 to 

 3.562. 



. Hence, when the Earth is in the aphelion, the 

 length of the Moon's shadow is 59.730 ; and if the 

 moon is in the perigee, its distance from the earth 

 is only 55.902 ; so. that the shadow may reach the 

 earth, and a total eclipse may take place. But if 

 the moon were in her apogee, when her distance 



VOL. II. K is 



