VOL. LXXI.] PHILOSOPHICAL TRANSACTIONS. 51 



orbit; the diurnal rotation round its axis; the menstrual motion round the com- 

 mon centre of gravity of the moon and earth; the precession of the equinoctial 

 points; the diminution of the obliquity of the ecliptic; the nutation of the 

 earth's axis: in short, every one of the motions that arise from the actions of 

 the sun, moon, and planets, combined with the spheroidical figure of the earth, 

 and the projectile and rotatory motions first impressed on it, have all been con- 

 sidered by astronomers, and their real and apparent inequalities investigated. 

 Aud to the great honour of modern astronomers it must be confessed, that no 

 science has ever made such considerable strides towards perfection in so short a 

 time as astronomy has done since the invention of the telescope. 



There is one of the motions of the earth however which, it seems, has hitherto 

 escaped the scrutiny of observers: viz. the diurnal rotation round its axis. The 

 principal reason why this has not been looked into, is probably the difficulty of 

 finding a proper standard to measure it by ; since it is itself used as the standard 

 by which we measure all the other motions. We have indeed no cause to sus- 

 pect any very material periodical irregularity, either diurnal, menstrual, or an- 

 nual; for the great perfection of our present time-pieces would have discovered 

 any considerable deviation from that equability which we have hitherto ascribed to 

 the diurnal motion of the earth. And yet, it is not perhaps altogether impos- 

 sible but that inequalities may exist in this motion, which, in an age when obser- 

 vations are carried to such a degree of refinement, may be of some consequence. 



To show how far time-keepers, though ever so perfect, are from being a 

 proper, or at least a sufficient standard, to examine the diurnal motion of the 

 earth by, it may be asked, whether it is probable, that any clock would have dis- 

 covered to us the aberration of the fixed stars? And yet that aberration produces 

 a change in longitude, and of consequence in right ascension, which causes an 

 annual irregularity in a star's coming to the meridian, which a time-piece, were 

 it a sufficient standard, would soon have discovered, and which we might have 

 attributed to an inequality of the earth's diurnal motion, had we not been ac- 

 quainted with its real cause. And if we were to find out any apparent irregu- 

 larity, acceleration, or retardation, should we not much rather suspect the clock 

 than the diurnal motion ? We may therefore venture to say, that the aberration 

 of the fixed stars, though attended with the above-mentioned consequence, 

 would for ever have remained a secret to us, had it not been found out by other 

 methods than time-keepers. 



Now, if time-pieces fail us in this critical case, where we stand in the greatest 

 need of their assistance, it is almost in vain to expect any help from another 

 quarter; for what mechanical movement on earth, or motion of the heavens, is 

 there that can measure out such equal portions of time as we require to compare 

 the diurnal motion of the earth to? However, to proceed, since we have 



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