UNITS AND STANDARDS OF MEASUREMENT. 385 



dimensions of space, their periods of revolution, their 

 changes of brilliancy and colours. It is obvious that 

 though astronomical numbers are conventionally called 

 constant, they are in all cases probably subject to more 

 or less rapid variation. 



Terrestrial Numbers. 



Our knowledge of the globe we inhabit involves many 

 numerical determinations, which have little or no con 

 nexion with astronomical theory. The extreme heights 

 of the principal mountains, the mean elevation of con 

 tinents, the mean or extreme depths of the oceans, the 

 specific gravities of rocks, the temperature of mines, all 

 the host of numbers expressing the meteorological or 

 magnetic conditions of every part of the surface must 

 fall into this class. Many of such numbers are hardly 

 to be called constant, being subject to periodic or even 

 secular changes, but they are no more variable in fact 

 than many which in astronomical science are set down 

 as constant. In many cases quantities which seem most 

 variable may go through rhythmical changes resulting 

 in a nearly uniform average, and it is only in the long 

 progress of physical investigation that we can hope to 

 discriminate successfully between those elemental num 

 bers which are absolutely fixed and those which vary. 

 In the latter case the law of variation becomes the 

 constant relation which is the object of our search. 



Organic Numbers. 



All the forms and properties of brute nature having 

 been sufficiently defined by the previous classes of numbers, 

 the organic world, both vegetable and animal, remains 

 outstanding, and offers a higher series of phenomena for 



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