[ ll S ] 
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 perfectly equable. In a Letter from 
Mr, William Herfchel of Bath to William Watfon, M, ZX 
E R, S, 
Read January xi, 1781. 
SIR, Bath, O&ober 18, T780. 
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- 
noftial points * the diminution of the obliquity of the ecliptic ; 
the nutation of the earth’s axis : in ftiort, 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 projeftile and rotatory motions firft imprefled upon it, 
have all been confidered by aftronomers, and their real and 
apparent inequalities inveftigated. And to the great honour of 
modem aftronomers it mu ft be confeffed, that no fcience has 
ever made fuch confiderable ftrides towards perfe&ion in fo 
Ihort a time as aftronomy has done fince the invention of the 
telefcope. 
There is one of the motions of the earth however which, 
it feerns, has hitherto efcaped the fcrutiny of obfervers ; I 
mean the diurnal rotation round its axis. The principal reafon 
Qj2 why 
