966 



SCIENCE. 



[N. S Vol. XV. No. 390. 



Ihoug-h too minute to be appreciable in 

 such intervals as are known to human his- 

 tory, must certainly become considerable in 

 the course of terrestrial history. Thus, 

 the contraction of the earth due to secular 

 loss of beat tends to shorten the day, while 

 accumulations of meteoric dust and tidal 

 friction tend to lengthen it.* There ex- 

 ists also a graver source of disturbance in 

 the slow rising and sinking of the crust of 

 the earth in different latitudes so often 

 pointed out by geologists. Such move- 

 ments are only partly compensating in 

 their effects on the day, and it seems 

 highly probable that they may cause irreg- 

 iilarities amounting to a few seconds in a 

 century -without entailing any noteworthy 

 fluctuations of the relative positions of the 

 land and sea.f 



It appears, then, that our time unit is the 

 least stable of the three fundamental units 

 and hence the most in need of checks on 

 its stability. Various other standards of 

 time have been proposed, but none of them 

 meets the requisites of permanency and 



* I have discussed the effects of secular cool- 

 ing and meteoric dust on the length of the day 

 in a paper published in the Astronomical Journ-al, 

 Vol. XXI., 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 his- 

 tory (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 accumu- 

 lations 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 num- 

 ber of the Astrono7nical Journal (Vol. XXII., 

 No. 11), 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. 



t See ' Mathematical and Physical Papers of 

 Lord Kelvin,' Vol. III., pp. 333-335, Cambridge 

 University Press, London, 1890. 



availability. The interests of astronom- 

 ical science especially demand that efforts 

 be made to find in the solar system some 

 better timekeeper than the earth. Possi- 

 bly the fifth satellite of Jupiter may serve 

 as a control on the constancy of rotation of 

 the earth. 



Turning now to a consideration of the 

 more complex quantities which are ex- 

 pressed in terms of length, mass and time, 

 we enter the boundless fields of physical 

 science in which measurement and calcula- 

 tion have revealed to us all ranges of mag- 

 nitudes from the vanishingly small to the 

 indefinitely large. It is in these fields that 

 we learn something definite concerning the 

 limitations of our senses; for while meas- 

 urements alone carry us but a little way 

 along lines of research, calculation dis- 

 closes not only the unseen, but also, in 

 many cases, phenomena which are quite 

 beyond the reach of any direct sense per- 

 ception.* 



To begin with quantities near the lower 

 limit of determination, think, for a mo- 

 ment, what is going on in the air which for 

 the present is the main medium of commu- 

 nication between us. No one has ever seen 

 the particles of the atmosphere in the sense 

 that we have all seen the particles, or eor- 

 X)uscles, of the blood. But we probably 

 Jmow more about the molecules of gases 

 than we do about blood corpuscles. By 

 actual count it is known that there are 

 four to six millions of the latter in a cubic 

 millimeter; and with equal definiteness 

 calculation shows us that there are about a 

 million million million molecules in a cubic 



* The reader may be referred to a very instruc- 

 tive paper by Dr. G. Johnstone Stoney entitled 

 ' Survey of that Part of the Range of Nature's 

 Operations which Man is Competent to Study.' 

 Scientific Proceedings of the Boi/al Dublin So- 

 ciety, Vol. IX., No. 13; Philosophical Magazine, 

 Fifth Series, No. 294, November, 1899; published 

 also in Report of SmitJisonian Institution for 

 1899. 



