478 



NOTES. 



NOTE 111, p. 45. Penumbra. The shadow or imperfect darkness, which pre- 

 cedes and follows an eclipse. 



NOTE 112, p. 46. Synodic revolution of the moon. The time between two 

 consecutive new or full moons. 



NOTE 113p. 46. Horizontal refraction. The light, in coming from a celes- 

 tial object/^ bent into a curve as soon as it enters our atmosphere; and that 

 bending is greatest when the object is in the horizon. 



NOTE 114, p. 46. Solar eclipse. Let S, fig. 28, be the sun, m the moon, and 

 E the earth. Then a E b is the moon's shadow, which sometimes eclipses a 



Fig. 28. 



small portion of the earth's surface at e, and sometimes falls short of it. To a 

 person at e, in the centre of the shadow, the eclipse may be total or annular ; 

 to a person not in the centre of the shadow, a part of the sun will be eclipsed ; 

 and to one at the edge of the shadow there will be no eclipse at all. The 

 spaces P 6 E, P' a E, are the penumbra. 



Fig. 29. 



NOTE 115, p. 48. From the extremities, $c. If the 

 length of the line a b, fig. 29, be measured, in feet or 

 fathoms, the angles S b a, Sab, can be measured, and 

 then the angle a S b is known, whence the length of the 

 line S C may be computed, a S b is the parallax of the 

 object S ; and it is clear that, the greater the distance of S, 

 the less the base a b will appear, because the angle a S'b 

 is less than a S b. 



