76 
above named have all but provoked, “imagine a locust 
stridulating in the centre of a mass of iron one mile in all 
directions” (séc). The idea is charming, countrified, 
bucolic, but perhaps rather cold for the poor insect! “It 
is admitted he could be heard, and about sixteen times 
quicker than in the air... .” (the steps of this grand 
calculation must perforce be omitted). “The mass of 
iron thus displaced” (ze. by said locust) “ would weigh not 
less than 729,749,050,612 tons, and would be so moved by: 
the strength of the locust.” 
The thought is too tremendous! so, locust-like, I must 
cease to “stridulate,” lest I bring down the solar system 
in ruins on the heads of innocent humanity. 
W. H. STONE 
Vol. III. : P2/ze. 
(Breslau: J. U. 
Kryptogamen-Flora von Schlesien. 
Bearbeitet von Dr. J. Schroter. 
Kern.) 
Dr. Coun’s “ Cryptogamic Flora” is already so favour- 
ably known by the portions which have appeared, that 
the announcement of any subsequent part is sure to be 
received with satisfaction. The first part of the Fungi, 
by Dr. J. Schréter, is just issued, and consists almost 
entirely of an introduction of nearly 100 pages, carefully 
digested and summarised, concluding with the order of 
classification adopted. The three groups or primary 
divisions are :—(1) Myxomycetes; (2) Schizomycetes ; 
and (3) Eumycetes. The latter embraces the Chytridiei, 
Zygomycetes, Oomycetes, Protomycetes, Ustilaginei, 
Uredinei, Auriculariei, Basidiomycetes, and Ascomycetes, 
with an appendix for the incomplete Hyphomycetes, 
Tuberculariei, and Sphzropsidei. As the present part 
contains only a portion of the Myxomycetes, no opinion 
can be formed of the manner in which the foregoing 
skeleton will be filled up; but, as this portion is based 
upon Rostafinski’s monograph, no exception can be taken 
thereto. The real difficulty lies further ahead, and 
whether the knot is to be untied or cut cannot be 
predicted. 
LETTERS TO THE EDITOR 
[Zhe Editor does not hold himself responsible for opinions expressed 
by his correspondents. Neither can he undertake to ieturn, 
or to correspona with the writers of, rejected manuscripts. 
No notice ts taken of anonymous communications. 
[The Editor urgently requests correspondents to keep their letters 
as short as possible, The pressure on his space is so great 
that it is impossible otherwise to insure the appearance even 
of communications containing interesting and novel facts.] 
On the Terminology of the Mathematical Theory of 
Elasticity 
I HAVE been greatly interested by the letters on this subject 
from Prof. K. Pearson (NATURE, vol. xxxi. p. 456) and Prof. 
A. B. W. Kennedy (vol. xxxi. p. 504), and I have looked 
forward with pleasure to further communications from other 
eminent ‘‘elasticians.” As, however, no better qualified person 
seems -disposed to continue the correspondence, and as I am 
practically interested in a definite settlement of elastic termin- 
ology, I venture to offer a few remarks on the subject. 
(1) Nothing could be better than Prof. Pearson’s term state of 
ease for the condition of an elastic body when capable of enduring 
a certain amount of stress, without showing permanent set on its 
removal. This is worthy of Clifford, and is sure to make its 
way. 
Prof. Kennedy has extended this term by applying ‘‘ maximum 
state of ease” to the condition in which the body may be 
strained to its elastic limit without set. Perhaps w/tzmate state of 
ease would be more significant, and /imited state of ease might be 
employed to denote the intermediate stages. The ultimate 
state of ease of course corresponds to the ‘‘ natural state” of the 
ideal perfectly elastic solid. 
At the point 4 in Prof. Kennedy’s figure we reach the Zit 
of perfect elasticity, and enter the stage 6 of elastic instability. 
Prof. Kennedy’s suggestion of ‘‘limit of stability” for the point 
NATURE 
— 
[ ay 28, 1885 
C is inconsistent with the last. I would suggest e/astic crisis as 
an alternative for ‘‘ breaking-down point.” We evidently here 
pass the critical point in the static equilibrium of the molecules. 
Perhaps c’ might be called the stage of thermal inversion, 
At C, the bar enters the plastic state—divided by Prof. 
Kennedy into the stage of uniform flow from C, to the point D 
of maximum load and the stage of local flow from D to the point 
E of terminal load or (apparently) of maximum stress. 
(2) I observe that Prof. Kennedy uses ‘‘load” and ‘‘ external 
stress,” apparently as alternative terms, and that Prof. Pearson 
speaks of ‘‘stress per unit area.” Would it not be advisable to 
settle, once for all, that s¢vess shall always, when it stands alone, — 
mean a force per unit area? ‘‘Stress” and “intensity of © 
stress ” would then be identical terms, while the force across a 
given area due to stress would be known as the ‘‘total” or 
‘resultant stress” across the area. This is all that is required 
to bring the terminology of Zerfect elasticity into exact corre- 
spondence with that of hydromechanics, in which pressure and — 
total or resultant pressure have always stood in this relation to 
one another. 
(3) Next as to ‘‘tension.” The word was originally adopted 
from the theory of strings, and of bars used like strings to sup- 
port weights, and was, I believe, invariably used (as it still is 
in the case of strings) to denote the load, or /ofa/ longitudinal © 
stress endured. Nowadays, however, it seems to be employed — 
indifferently in this sense and in that of intensity of tensile stress. 
I would suggest that the term ¢vaction, which the modern French 
writers have freely adopted, should be invariably used to denote 
intensity of tensile stress, and that ¢evszon should be restored to . 
its original signification of total or resultant traction. : 
‘* Traction” and ‘‘ pressure” would then (according to the 
ordinary convention as to sign) be synonymous with “‘ positive ” 
and ‘‘ negative” stress. Perhaps some ela-tician would suggest © 
a convenient abbreviation for ‘‘total pressure” or ‘‘negative — 
tension.” 
(4) Is it too late to protest on behalf of that much-abused 
term véscostty as applied to solids? The thoroughly-established — 
sense of the word, as applied to fluids, implies—vof the property — 
in virtue of which they undergo permanent or continued change ~ 
of shape under continued distorting stress (z.e. their futdzty) ; 
but that other property in virtue of which they are able to 
offer more or less resistance, by means of molecular friction, to 
instantaneous changes of shape under stress which is mot con-— 
tinued. In this case, therefore, viscosity isa property distinctly _ 
opposed to fluidity, and, indeed, described in terms as a falling 
short of ‘‘ perfect fluidity.” ; 
It is thus obviously false analogy to describe a metal in the 
state of plastic flow as vzscows, or to ‘‘ appropriate this name to 
that permanent set which may be produced by the ancl 
for a long period of a stress well within the limits of elasticity.” 
The latter sense—at Jeast as applied to ice (NATURE, vol. xxxil. 
p- 16)—has, no doubt, a classical authority in the great memoir 
of Forbes ; but Sir W. Thomson has pointed out (‘‘ Enc. Brit.,” 
Art. ‘‘Elasticity,” § 31; and Thomson and Tait’s ‘‘ Natural 
Philosophy,” § 741) that the properties of ice so described ar 
included under the perfectly definite and convenient term 
Plasticity, which is really analogous to fluidity. 
On the other hand, analogy demands that the term wiscoszty, 
as applied to solids, shall be strictly confined to that frictional 
dissipation of energy which is always at work during rapi 
changes of strain, and which was first discovered during small 
vibrations within the elastic limit by Sir W. Thomson (Proc. 
Roy. Soc., May 18, 1865, or the passages above cited). 
That the viscosity of a ductile material is very greatly increased 
in the plastic stage is of course beyond a doubt, the amount o} 
energy absorbed by it on sudden increase of the stress being s 
much in excess of that required to provide for the increas: 
potential energy of the accompanying strain that the temperatur 
rises to a sensible extent. But what I wish to make clear is tha! 
the true viscosity is not essential to or characteristic of the truly 
plastic state, but that, on the contrary, the viscosity of a ductil 
solid renders it zvzferfectly plastic in just the same sense as 
viscous fluid is zwperfectly fluid. 
(5) Finally, I may perhaps be permitted to add that, next t 
the importance to all concerned of a definite and univers: 
terminology, comes the importance to mathematicians at least o! 
a uniform sofation. , 
The effect of reading through, for purposes of comparison o 
historical record, the 100 odd veadly important treatises on this 
subject—in each of which a perfectly independent and generall 
