700 
such, for instance, as magnolia at Glouces- 
ter, and great rose-bay at Sebago, — may be 
stripped of their treasures. These ‘ late-lin- 
gerers’ possess great interest, and they should 
long be carefully guarded. But, so far as our 
rare plants in general are concerned, we do 
not yet need any society for their preserva- 
tion: we do, however, need many local soci- 
eties for their detection, and for critical study 
of their habits. 
A. YEAR ago five commissioners of state 
water-supply were appointed by the New- 
Jersey legislature to select the best practicable 
plans for supplying the cities and towns of the 
state with pure and wholesome water. A report 
has recently been presented by them to the gov- 
ernor, on the capabilities of the Passaic-River 
basin for the collection and storage of water 
for the several centres of population that must 
now, or in the near future, depend uponit; and 
a plan elaborated by Mr. L. B. Ward, hydraulic 
engineer, is appended for the supply of Jersey 
City, Newark, and other neighboring munici- 
palities. . According to this plan, the waters 
of the Pequannock, a tributary of the Passaic, 
can furnish sixty million gallons daily, at an 
expense of two million dollars. With a further 
cost of three hundred thousand dollars, the 
supply can be increased to eighty million gal- 
lons, sufficient for all probable requirements 
fortwenty yearsto come. Farther inthe future, 
the Wanaque and Ramapo watersheds can 
yield an additional two hundred million gallons 
daily, so as to serve a population of two mil- 
lion eight hundred thousand souls. The chief 
danger of pollution in the Pequannock valley 
is of a modern kind: it comes from leakage of 
the Oil transit company’s pipes that carry pe- 
troleum from the oil-wells of Pennsylvania to 
Jersey City ; but this danger can be averted by 
state enactment. Mr. Ward’s report contains 
a well-prepared contour-line map of the Pe- 
quannock basin, with darker and darker tints 
for every elevation of one hundred feet: this is 
reproduced from a more extended map, based 
on ‘ the valuable contoured maps of the New- 
Jersey geological survey,’ and on special sur- 
a) ale is) ele Pitta! neh Sts, 
SCIENCE. 
[Vou. III., No. 71. 
veys by the commission in the adjacent part 
of New York. In view of the rapid growth of 
many of our cities, and of the increasing rec- 
ognition of the value of a good water-supply, 
this fore-thoughtful action of the New-Jersey 
legislature should be imitated in other states. 
LETTERS TO THE EDITOR. 
x*, Correspondents are requested to be as brief as possible. 
The writer’s name is in all cases required as proof of good faith. 
Professor Tait on the reality of force. 
THE arguments by which Professor Tait seeks to 
disprove the objective reality of force, and to justify 
his advocacy of the exclusion of the term from scien- 
tific writing, occupy two and a half pages at the end 
of a seventy-four page article on mechanics, in the 
last edition of the Encyclopaedia Britannica. The 
vigor and confidence with which they are there 
stated, notwithstanding the author’s treatment of 
forces as real entities in the body of the article, the 
character of the publication in which they appear, 
and the eminence of the Edinburgh professor in 
mathematics and physics, make them worthy of a 
careful examination. 
In the first place, Professor Tait infers that force 
can have no such reality as matter has, because it is 
to be reckoned positively and negatively, —an action 
being opposed by a reaction, — while matter, or mass, 
is signless. This suggests two comments: 1°. The 
author never questions the objective reality of space 
and time, of which realities it is an essential feature, 
that, to every direction or interval A-—B, an equal 
direction or interval B~A, of opposite sign, corre- 
sponds; 2°. The idea of a negative mass is not a self- 
contradictory one, and was once widely accepted. 
The element phlogiston was given up, not because of 
any absurdity in ascribing levity to material sub- 
stance, but because a form of matter with positive 
mass (oxygen), capable of explaining all the phe- 
nomena, had been actually separated and identified. 
Professor Tait’s next criterion of objective reality is 
quantitative indestructibility, — an attribute shared 
by time, space, and matter, to which he adds energy. 
But the evidence of the indestructibility of energy 
is not of the same nature as that of the indestructi- 
bility of matter: for the latter, in all its forms, may 
be localized, and its density or elasticity measured; 
while the former, when stored up or ‘ potential,’ can- 
not be shown to possess a single one of the properties 
of energy kinetic, or any existence in space, or any 
objective character whatever. Professor Tait virtu- 
ally admits this difficulty, and awaits for its solution 
the discovery of some evidence ‘as yet unexplained, 
or rather unimagined.’ All strains and other actions 
of a clock-weight on its supports are obviously pre- 
cisely the same — or impalpably somewhat stronger— 
with the weight wound up an inch as with it wound 
up a yard; and the existence of a greater ‘ potential 
energy ’ in the latter case is not to be found in the 
clock, but in the mind, which requires this expression 
as a form in which to put its conviction that a cer- 
tain greater amount of work can be obtained. Even 
though it be admitted that there are no other intel- 
ligible terms in which this conviction can be stated, 
it is clear that the indestructibility of energy is an 
ideal and subjective truth, and cannot, therefore, be 
relied on as evidence of a reality distinctively ‘ objec- — 
tive. . 
