of the earth viewed from the moon. 245 
described will be an oval, whose transverse axis lies SW and NE, 
when the apogee is nearest the ascending node, and NW and SE when 
it is nearest the descending node, and more or less eccentric as the 
apogee is nearer to, or farther from either node ;_ and the direction 
when the apogee is in north latitude is. by the north to the east, and 
when in south latitude by the south to the east. 
The numerous irregularities of the moon’s motion will produce 
various waving deviations of 1° or 2° from the path above described, 
which i is the one that would result from the two causes contemplated, 
and the mean of all. the others. 
The time from the parting i moeeed aes aaiets 
their meeting again is at a mean one or two days less than six years. 
In this time there are 793 librations in longitude and 803 in latitude. 
Mr. Ferguson supposes the lunarians may measure time by the 
motion of the earth on its axis, and the libration of its poles, but would 
either of them be as much noticed there, as the motion just now ex- 
_ amined, of which the period is 73 or 74 lunar days ? As the sun 
crosses this parallelogram every lunation, the path may be it investigat- 
ed by them for the purpose of computing eclipses. = 
This cycle of motions, various as they are, may be easily ienitated 
by a pendulum, formed by a heavy ball C, Fig. 14, and the flexible 
thread BC suspended on two branches of the same thread DB, BE, 
of such lengths, that by continuing CB to meet DE in A the line AB 
may be 3 2, of AC ; these branches being in the plane of the meridian. 
If this pendulum be. impelled SW, in about twenty vibrations they 
will gradually expand into a circle in the direction of the sun’s diur- 
nal motion, In twenty more they will shrink into a right line lying 
‘SEand NW. In twenty more they will expand intoa circle ina 
direction contrary to the former, and after the remaining twenty it will 
resume the motion et communicated. 
