303 APPENDIX. 



43, both the moon and sun tend to raise tides at the same 

 places as at a and h, 



LESSON 49. 



Eclipses, j/J'^Mrcs 44, and 45, Engr. VI. Fig. 45, repre- 

 sents an eclipse of the moon. A F and B G are two straight 

 lines drawn from the opposite parts of the solar disk, touch- 

 ing the surface of the earth at C and D. The moon m is 

 seen passing through the earth's shadow in opposition to the 

 sun. Besides the dark shadow of the earth, C F D G which 

 would terminate in a point if continued far enough, there is 

 another shadow C r s D, distinct from the former and called 

 the penumbra, which is faint at the edges towards r and S 

 but becomes darker towards F and G. The instant the 

 moon enters the earth's shadow at x, it is deprived of the 

 sun's light, and is eclipsed to all in the illuminated hemi- 

 sphere of the earth. As the shadow of the earth is but a little 

 darker than the region of the penumbra next it, it is difficult 

 to determine the exact time when the moon passes from the 

 penumbra into the shadow, and from the shadow into the 

 penumbra, that is, when the eclipse begins and ends. Fig- 



44, represents an eclipse of the sun. As the sun constantly 

 illuminates half the earth's surface, and as the moon's shadow 

 falls upon but a part of this illuminated hemisphere, the sun 

 therefore appears eclipsed to but a part of those to whom he 

 is visible. Sometimes when the moon is at its greatest dis- 

 tance, its shadow o m terminates before it reaches the earth, 

 and then to an inhabitant directly under the point o, the 

 eclipse will appear annular. The other shadow C r s D is 

 the penumbra. Within the dark shadow, the sun is totally 

 eclipsed, but within the penumbra, only a part of the sun's 

 rays are intercepted, and the sun is partially eclipsed. The 

 beginning and ending of a solar eclipse may be determined 

 instantaneously. The penumbra, under the most favourable 

 circumstances, falls upon but about half of the illuminated 

 hemisphere of the earth. Note. The pupil should be taught 

 to enlarge the above as well as other explanations of the 

 figures. 



LESSON 60. Caloric. 

 Different kinds of Thermometers. Fahrenheit's thermo- 

 meter is universally used in Great Britain, and for the most 



