xiv.] UNITS AND STANDARDS OF MEASUREMENT. 333 



changes of brilliance and colour. It is obvious that 

 though astronomical numbers are conventionally called 

 constant, they are probably in all cases 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- 

 nection with astronomical theory. The extreme heights 

 of the principal mountains, the mean elevations of 

 continents, the mean or extreme depths of the oceans, 

 the specific gravities of rocks, the temperature of mines, 

 the host of numbers expressing the meteorological or 

 magnetic conditions of every part of the surface, must 

 fall into this class. Many such numbers are not to be 

 called constant, being subject to periodic or secular 

 changes, but they are hardly more variable in fact than 

 some 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 uetween those elemental num- 

 bers which are 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. 



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 higner series of phenomena for 

 our investigation. All exact knowledge relating to the 

 forms and sizes of living things, their numbers, the 

 quantities of various compounds which they consume, 

 contain, or excrete, their muscular or nervous energy, &c. 

 must be placed apart in a class by themselves. All such 

 numbers are doubtless more or less subject to variation, 

 and but in a minor degree capable of exact determination. 

 Man, so far as he is an animal, and as regards his physical 

 form, must also be treated in this class. 



