30 RECORDS 



in the slow rising and sinking of the crust of the earth in different 

 latitudes so often pointed out by geologists. Such movements 

 are only partly compensating in their effects on the day, and it 

 seems highly probable that they may cause irregularities amount- 

 ing to a few seconds in a century without entailing any note- 

 worthy fluctuations of the relative positions of the land and sea.^ 



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 pro- 

 posed, but none of them meets the requisites of permanency 

 and availability. The interests of astronomical science espe- 

 cially demand that efforts be made to find in the solar system 

 some better timekeeper than the earth. Possibly the fifth satel- 

 lite of Jupiter may serve as a control on the constancy of rota- 

 tion of the earth. 



Turning now to a consideration of the more complex quanti- 

 ties which are expressed in terms of length, mass and time, we 

 enter the boundless fields of physical science in which measure- 

 ment and calculation have revealed to us all ranges of magnitudes 

 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 measurements alone carry 

 us but a little way along lines of research, calculation discloses 

 not only the unseen, but also, in many cases, phenomena which 

 are quite beyond the reach of any direct sense perception.^ 



To begin with quantities near the lower limit of determination, 

 think, for a moment, what is going on in the air which for the 

 present is the main medium of communication between us. No 

 one has ever seen the particles of the atmosphere in the sense 

 that we have all seen the particles, or corpuscles, of the blood. 

 But we probably know more about the molecules of gases than 



1 See " Mathematical and Physical Papers of Lord Kelvin," Vol. Ill, pp. ^^j- 

 335, Cambridge University Press, London, 1890. 



^ The reader may be referred to a very instructive 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 Pi-oceedings of the Royal Driblin Society, 

 Vol. IX, No. 13 ; Philosophical Magazine, Fifth Series, No. 294, November, 1S99; 

 published also in Report of Smithsonian Institution for 1899. 



