958 
’Proceedings of the Royal Roc'ety 
This element, however, does not of course enter into the cost of 
production, which involves merely the apparatus, mediate products 
and energy actually expended, plus those losses by fire, &c., already 
mentioned. And thus, though the net quantity of ultimate product 
may be very small in relation to the gross quantity exploited, the 
process yields a profit so long as the sum of the rectangles repre- 
senting the cost of production at each stage does not equal or exceed 
that representing ultimate products, and this surplus is in fact the 
interest paid by Nature upon the matter and energy expended upon 
her during the processes of production. The rationale of the partition 
of this surplus, and of the ultimate products generally, cannot be 
considered until after the psychological and social conditions, which 
so largely determine them, have been investigated ; the actual facts 
of movement and position of ultimate products can of course be 
observed, and the physical economist then has means of measure- 
ment and comparison other than that afforded by money, — his 
units are really the metre and kilogram of the physicist, — and only 
when some quantitative data in these terms are approximately 
reached will those expressed in money be really capable of inter- 
allowed, the difficulty of such statistical 
measurement of all the greater economic 
processes might be shown to be much 
more apparent than real. Many frag- 
mentary investigations of this kind indeed 
already exist ; thus we constantly find the 
mechanical energy of Britain described 
as the addition of so many million 
workers; suggestions, too, have constantly 
been made to estimate capital, or to ex- 
press the ratio of exchange of products in 
terms of the labour they represent, and 
many other ideas emitted which await the systematic application of 
physical measurements. 
§ 19. Quantity of Ultimate Products i:>er unit time . — The quantity 
of ultimate products obtained per unit time being known, the average 
store of wealth of the community, and even the details of partition 
are readily observed and denoted. Thus (fig. 4) the rectangle 
ABCD may denote the average wealth of a community at one time, 
pretation. And if space 

Fig. 4. 
