VOL. XLVI.] PHILOSOPHICAL TRANSACTIONS. 17 



from that to our time ; because the course of the Julian years, according to 

 which every 4th ought to have been bissextile, has been frequently interrupted 

 by the pontifices ; of which we find some sure marks in Censorinus and Dion 

 Cassius. Hence it might well happen, since the times marked by Ptolemy, that 

 there has really been a day or two more than we reckon, and consequently, that 

 Ptolemy's equinoxes, ought to be put a day or two back ; which would lengthen 

 the years of those times. I was in hopes that the Arabian observations would 

 not be liable to this inconvenience; because the Julian calendar has not been 

 interrupted for these last past 1200 years. The late Dr. Halley had also re- 

 marked, that the revolutions of the moon are quicker at present than they were 

 in the time of the ancient Chaldeans, who have left us some observations of 

 eclipses. But as we measure the length of years by the number of days, and 

 parts of a day, which are contained in each of them ; it is a new question, whe- 

 ther the days, or the revolutions of the earth round its axis, have always been of 

 the same length. This is unanimously supposed, without our being able to pro- 

 duce the least proof of it : nor indeed do I see how it could be possible to per- 

 ceive such an inequality, in case it had really existed. At present we measure 

 the duration of a day by the number of oscillations, which a pendulum of a given 

 length makes in this space of time : but the ancients were not acquainted with 

 these experiments, by which we might have been informed, whether a pendulum 

 of the same length made as many vibrations in a day foraierly as now. But even 

 though the ancients had actually made such experiments, we could draw no in- 

 ferences from them, without supposing, that gravity, on which the time of an 

 oscillation depends, has always been of the same force : but who will ever be in 

 a condition to prove this invariability in gravity ? thus, even supposing that the 

 days had suffered considerable changes; and that gravity had been altered suitably 

 to them, so that the same pendulum had always completed the same number of 

 vibrations in a day ; it would nevertheless be still impossible for us to perceive 

 this inequality, were it ever so great. And yet I have some reasons, deduced 

 from Jupiter's action on the earth, to think, that the earth's revolution round 

 its axis continually becomes more and more rapid. For the force of Jupiter so 

 accelerates the earth's motion in its orbit round the sun, that the diminution of 

 the years would be too sensible, if the diurnal motion had not been accelerated 

 nearly in the same proportion. Therefore, since we hardly at all remark this 

 considerable diminution in the years, from thence I conclude, that the days 

 suffer much the same diminution ; so that the same number will answer nearly 

 to a year. 



VOL. X. D 



