j 1 6 Mr. herschei/s Agronomical Obfrrvations 
why this has not been looked into, is probably the difficulty 
of finding a proper ftandard to meafure it by ; fince it is itfelf 
ufed as the ftandard by which we meafure all the other moticns.- 
We have, indeed, no caufe to fofpeft any very material perio- 
dical irregularity, either diurnal,' menftrual, or annual ; for 
the great perfection of our prefent time-pieces would have dis- 
covered any confiderable deviation from that equability which i 
we have hitherto afcribed to the diurnal motion of the earth. 
And yet, it is not perhaps altogether impoflible but that : 
inequalities may exift in this motion which, in an age where ; 
obfervations are carried to fuch a degree of refinement, may 
be of fome confequence. 
To ftiew how far time-keepers, though ever fo perfeCl, are* 
from being a proper, or at leaf! a fufficient, ftandard to exa- • 
mine the diurnal motion of the earth by, I may afk, whether 
it is probable, that any clock would have difcovered to us the 
aberration of the fixed ftars ? And yet that aberration produces 
a change in longitude, and of confequence in right afcenfion,. 
which caufes an annual irregularity in a ftar’s coming to the 
meridian, which a time-piece, were it a fufficient ftandard, j 
would foon have difcovered, and which we might have attri- 
buted to an inequality of the earth’s diurnal motion, had we 
not been acquainted with its real caufe. And if we were to 
find out any apparent irregularity, acceleration, or retardation, 
fhould we not much rather fufpeft the clock than the diurnal 
motion ? I may therefore venture to fay, that the aberration oft 
the fixed ftars,- though attended with the above mentioned con- 
fequence, would for ever have remained a fecret to us, if it 
had not been found out by other methods than time-keepers* 
Now, if time-pieces do fail us in this critical cafe, where 
we ftand in the greateft need of their affiftance, it is almoft in 
vain 
