1. 1 6 Mr. herschel’s Agronomical Obfervations 
why this has not been looked into, is probably the difficulty 
of finding a proper ftandard to meafure it by ; fince it is itfelf 
tifed as the ftandard by which we meafure all the other motions. 
We have, indeed, no caufe to fufpeft any very material perio- 
dical irregularity, either diurnal, menftrual, or annual ; for 
the great perfection of our prefent time-pieces would have dif- 
covered any confiderable deviation from that equability which 
we have hitherto afcribed to the diurnal motion of the earth* 
And yet, it is not perhaps altogether impoffible but that 
inequalities may exift in this motion which, in an age where 
©bfervations are carried, to fitch a degree of refinement, may 
be of fome confequen.ce. 
To fhew how far time-keepers, though ever fo perfect, are 
from being a proper, or at leaft a fufficient, ftandard to exa- 
mine the diurnal motion of the earth by, I may aik, whether: 
it is probable, that any clock would have dilcovercd 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,. 
would foom have di (covered, 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,. 
Chould we not much; rather fufpeft the clock than the diurnal: 
motion ? I may therefore venture to fay, that the aberration of 
the fixed (tars, 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 hand in the greajeft need of their affiftance, it is almoft in 
vain.. 
