THE RATE-BASE AND DEPRECIATION S51 
law and policy seems to require the allowance of a reasonable 
return upon at least that sum. 
“ Upon the present trial, it was insistently urged upon me by 
some of the defendants that there should be deducted from the 
cost of the property (irrespective of whether ‘original,’ ¢ pre- 
war,” or ‘ present reproduction ’ cost be under consideration) an 
amount claimed to represent so-called ¢ accrued theoretical de- 
preciation,” based upon an assumption of ‘life expectancy ’ for 
a gas plant and equipment and the estimated or known number 
of years since the same was erected or installed. From the 
testimony given upon the trial, I was strongly impressed that, 
in respect of a very large proportion of gas property, there is no 
ascertainable ‘life expectancy.” The withdrawal of such prop- 
erty from service comes about from inadequacy or obsolescence, 
which cannot be forecast in terms of years or even satisfactorily 
guessed at. 
“ Certain parts of operating machinery and equipment are of 
course subject to the effects of use. The replacement of these 
wearing parts enters into the cost of repairs. As to the sub- 
stantial units of structures, apparatus, mains, and equipment, 
their withdrawal from the property accounts comes about from 
causes not attributable to the condition of the property itself, 
or any diminution in its operating efficiency, but varying utterly 
with the particular plant, time, local conditions, and service 
demands, and hence capable of being forecast only as the occa- 
sion for such change in plant or equipment becomes imminent. 
“ In other words, in order to keep abreast of improvements 
in the art of making and distributing gas when and as it becomes 
economically advantageous to do so, and to meet the growing 
demand of the public for service more adequately and economic- 
ally than would be possible through merely making additions and 
extensions to existing plant and equipment, larger or better and 
more economical and efficient units of plant and equipment are 
from time to time installed, to take the place of units which are 
still operating as efficiently as when first installed. The loss due 
to such supersession cannot properly be said to have accrued 
during the period the superseded unit was in service. It occurred 
when supersession took place. It became a proper charge against 
the economies to be realized therefrom. It furnished no basis 
for the imposition of an additional charge against the user of the 
superseded unit during the period of its useful service, over and 
above the higher cost of operating it. Such a charge could not 
be justified, either on the ground that the unit was losing poten- 
Pa