RECORDS 29 



of mass must be regarded as less secure than our standard of 

 length, although the prototype kilograms are less likely to 

 change in mass with the lapse of time than the prototype 

 meters are to change in length ; for while such a general varia- 

 tion in volume as is known to occur in metals, especially alloys, 

 need not affect the former, it would almost certainly affect the 

 latter. 



Our unit of time is also known with a definiteness that meets 

 in most cases the highest demands of science at the present 

 epoch. The period of rotation of the earth, or the sidereal day, 

 is the standard interval of time, though it has been found con- 

 venient for many purposes to use the shorter interval of a mean 

 solar second, of w^hich there are 86,164.1 in a sidereal day. 

 That the earth rotates with wonderful regularity is a fact of the 

 highest importance to science. Without that regularity the 

 development of sidereal and planetary astronomy, with all they 

 have entailed, would have been impossible except by the dis- 

 covery of some other equally trustworthy timekeeper. But the 

 laws of mechanics, which show us plainly why the earth rotates 

 with such remarkable regularity, also show us that its period 

 of rotation is subject to sources of disturbance, some tending to 

 increase and some tending to decrease that period, whose effects, 

 though too minute to be appreciable in such inten^als as are 

 known to human history, must certainly become considerable 

 in the course of terrestrial history. Thus, the contraction of 

 the earth due to secular loss of heat tends to shorten the day, 

 while accumulations of meteoric dust and tidal friction tend to 

 lengthen it.^ There exists also a graver source of disturbance 



' I have discussed the effects of secular cooling and meteoric dust on the length 

 of the day in a paper published in the Astrottomical Journal, Vol. XXL, No. 22, 

 July, 1901. From this paper it appears that the change in length of the day from 

 secular cooling cannot be perceptible during any such brief interval as that of human 

 history (twenty centuries, say); but that in the course of complete cooling, or in a 

 million million years, say, the change in length of the day may amount to as much 

 as six per cent, of its original length. 



From the same paper it appears that accumulations of meteoric dust will only 

 begin to be perceptible in their effects on the length of the day when the process of 

 secular cooling has been substantially completed. In a subsequent number of the 

 Astronomical Journal (Vol. XXII., No. II ), Dr. G. Johnstone Stoney has shown 

 that if the compression produced by a layer of meteoric dust is taken into account 

 the effect will be still less than that just indicated. 



