E C L 



In the earth's orbit h is very small, .*. e | h nearly. 



The secular diminution 18". 79, and .". if this diminution continued 

 uniform (which, however, we have not a right to suppose) the earth's 

 orbit would become a circle in about 36300 years, 



ECHO. 



That an echo may return one syllable as ?oon as it is pronounced, the 

 reflecting surface should be 80 or feet distant ; for a dissyllablic echo 

 170 feet, &c. This is upon the supposition that sound proceeds at the 

 rate of 1142 feet per second, and that the ear can distinguish the succes- 



sion of two sounds or syllables, when the interval between them is th 

 of a second.. (Play fair.) 



An echo in Woodstock Park repeats 17 syllables by day, and 20 by 

 night. An echo on the north side of Shipley church in Sussex, repeats 

 21 syllables. (Young's Nat. Phil.) 



ECLIPSES. ( Woodliouse, Play fair.) 



\. Eclipses of the Moon. 



1. The length of the earth's shadow varies, according to the distance of 

 the sun and earth, between the limits of 212,896, and 220,238 semidiame- 

 ters of the earth j its mean length being 216,531. And in general if r be the 



earth's radius, - the apparent semidiameter, and p the horizontal par- 

 allax of the sun, the length of the shadow, reckoned from the earth's 

 centre, 



~ sin. (R'-* sin. JJ 



\ 2 P ' 220 



2. Hence half the angle subtended at the earth's centre by the section 

 -of the shadow, at the distance of the moon, (if P be the horizontal paral- 

 lax of the moon) is 



From this formula the apparent diameters of the earth's shadow may 

 be computed for various distances of the sun and moon, as in the follow- 



ing Table. 



Apparent diam. of 



earth's shadow. 

 r Moon in apogee ... .............. 1. 15'. 24".3036 



Sun in perigee ......... < at mean distance ................. 1. 23. 2'31 



tin perigee ....... ................ tf 1. 30. 40.3164 



