t II 5 ] 
X. Agronomical Obfervations on the Rotation of the Planets 
round their Axes , made with a View to determine whether the 
Earth's diurnal Motion is perfeBly equable . In a Letter from 
Mr. William Herfchel of Bath to William Watfon, M. D. 
F. R. X 
Read January ii, 1781. 
SIR, Bath, Odtober 18, 1780. 
T HE various motions of the planet we inhabit ; the an- 
nual revolution in its orbit ; the diurnal rotation round 
its axis ; the menftrual motion round the common center of 
gravity of the moon and earth ; the preceffion of the equi- 
nodtial points j the diminution of the obliquity of the ecliptic ; 
the nutation of the earth’s axis : in fhort, every one of the 
motions that arife from the adtions of the fun, moon, and 
planets, combined with the fpheroidical figure of the earth, 
and the projedtile and rotatory motions firft impreffed upon it, 
have all been confidered by aflronomers, and their real and 
apparent inequalities invefligated. And to the great honour of 
modern aflronomers it muff be confeffed, that no fcience has 
ever made fuch confiderable ftrides towards perfection in fo 
fliort a time as aftronomy has done fince the invention of the 
telefcope. 
There is one of the motions of the earth however which, 
it feems, has hitherto efcaped the fcrutiny of oblervers ; I 
mean the diurnal rotation round its axis. The principal reafon 
Qj2 why 
