NATURE 
[March 24, 1870 
LE FIMMA KES NON Se IR JAIOWMINOV 
[The Editor does not hold himself responsible for opinions expressed 
by his Correspondents. No notice is taken of anonymous 
communications. | 
Rotation of a Rigid Body 
My previous communication about the rotating ellipsoil to 
this Journal, has attracted the attention of M. Radau. ‘‘One 
touch of ature makes the whole world kin.” Ina note ad- 
dressed to me full of true dignity, this gentleman has made much 
more than sufficient reparation for his previous trifling act of inad- 
vertence, and states that to his great regret he had misunderstood 
my meaning, in the passage of my memoir in question, and that “sa 
critique n’est pas fondée.” I, on my part, deeply lament the un- 
necessary tone of acerbity in which my reference to this criticism 
was couched, and wish I could recall every ungracious expression 
which.it contains. ** When I spoke that, I was ill-tempered too.” 
I will pass over this, to me, painful topic, to say two or three 
words on the mode in which the rotating ellipsoid may be sup- 
posed to roll or wodd/e on a rough plane, with its centre fixed, 
My solution may remind the reader of Columbus’s mode of sup- 
porting an egg on its point—or, rather, of a fairer mode which 
Columbus might have employed, and which would not have 
necessitated the breaking of the shell, viz., by resting the blade 
of a knife or rough plate on the upper end of his egg. 
So, to make an ellipsoidal or spheroidal top roll, with its 
centre fixed—say, upon a rough horizontal plane—imagine a 
second horizontal plane in contact with the upper portion of its 
surface ; then the line joining the two points of contact will pass 
through the centre of the top. We may conceive a slight per- 
foration in either or each plane at its initial point of contact with 
the top, and a screw wire introduced through this, and inserted 
into a female screw in the body to be set rolling (a mode of 
spinning which Sir C. Wheatstone recommends as the most 
elegant in any case, and in this case evidently the most eligible). 
On withdrawing the wire with a jerk, the top may be set in 
motion about its centre, in such a direction as to remain in 
contact with the two planes, and if these be sufficiently rough 
the motion will eventually be reduced to one of pure rolling 
between them, the axis (#.¢, the line joining the two points of 
contact) continually shifting, but the centre remaining absolutely 
stationary: for, vertical motion this point cannot have, so long 
as the top continues to touch both planes, and any slight hori- 
zontal motion (if it should chance to take on such at the outset) 
would be checked and ultimately destroyed by the friction, 
which would also keep the two points of contact stationary (like 
the single point of contact of a wheel rolling on a rail), in each 
successive atom of time. Thus the motion upon the lower 
plane would in the end be precisely the same as if the upper 
plane were withdrawn, and the centre of the top kept fixed 
by some mechanical adjustment. If the spin were not suffi- 
ciently vigorous, after a time the rolling top might quit the 
upper plane, and of course sooner or later by the diminution of 
the ws-eiva due to adhesion, resistance of the air, imperfection 
or deformation of the surfaces, and other disturbing causes, this 
would take place, but abstracting from these circumstances the 
principal axes of the spheroidal or ellipsoidal top would move 
precisely in place and time like the ‘axes of spontaneous 
rotation” of any free body of which the top was the “ Kinematic 
Ixponent.” 
I do not pretend to offer an opinion what materials for the 
planes and rolling body (ground glass and ebony or roughened 
ebonite have been suggested to me) it would be best to employ, 
or whether the “‘ wobbling top” could easily be made to exhibit 
its evolutions. It is enough fora non-effective, unpractical man (as 
unfortunately I must confess to being) to have shown that there is 
no intrinsic impossibility in the execution of the conception. 
With regard to the friction and pressure: if W be the weight 
of the body, F and P the friction and pressure in the case of a 
single plane (the values of which are set out in my memoir, 
pp. 764—766, ‘*Philosophical Transactions” 1866), it may easily 
be proved that eventually the friction at each point of con- 
2 P+W 
? 
: iE i 
tact will be 3 the pressure upwards at the lower point 
and downwards at the upper one 
, so that if P should 
become equal to W the top would quit the upper plane and the 
experiment come to an end. At p- 766 of my memoir the 
factor \/M_A has accidentally dropped out of the expression for 
P which I mention here, in case any one should feel inclined to 
consult the memoir in consequence of this note. Mr. Ferrers 
has taken up my investigations, and given more compendious 
expressions than mine for F and P; with the aid of these it 
would probably be not difficult to determine the maximum value 
of p 8° as to assign the necessary degree of roughness of the 
confining planes, and also to ascertain under what circumstances 
P— W would become zero, but I do not feel sufficient interest 
in the question, nor haye I the courage to undertake these 
calculations with the complicated forms of P and F contained in 
my memoir. Mr. Ferrers’ results are contained in a memoir 
ordered to be printed in the ‘ Philosophical Transactions,” and 
will shortly appear. 
In my memoir will be found an exact kinematical method of 
reckoning the time of rotation by Poinsot’s ellipsoid when the 
lower surface is made to roll on one fixed plane at the same 
time that its upper surface is shapened off in a particular way 
(therein described) so as to roll upon a parallel plane which 
turns round a fixed axis; this upper plane is compelled 
to turn by the friction, and acts the part of a moveable 
dial in marking the time of the free body imaginarily 
associated with the ellipsoid. I have also shown there that the 
motion of any free body about a fixed centre may be regarded 
as compounded of a uniform motion of rotation and the motion 
of a disc, or, if one pleases, a pair of mutually bisecting cross- 
wires left to turn freely about their centre. But I fear that 
NaTUuRE, used to a more succulent diet, has had as much as it 
can bear upon so dry a topic, and, although having more to say, 
deem it wiser to bring these remarks to an end. 
J. J. SYLVESTER 
“Dutch” or ‘‘Deutsch” 
THERE is a short note in Mr. Huxley’s lecture in the last 
number of NATURE, which I have read several times in the vain 
hope of finding out its meaning. Mr. Huxley speaks of “the 
much debated question, did the Germans of Czesar and Tacitus 
speak ‘Deutsch’ (not ‘ Dutch,’ pace Mr. Freeman) or Celtic.” 
What has my ‘‘ peace,” or anybody else’s peace—save, perhaps, 
the Pax Romana—to do with it ? 
I do not see why Mr. Huxley brings in my name. He can 
hardly suppose that I do not know that Deuésch is the German 
form—I can hardly suppose that he does not know that Duich 
is the English formi—of the name otherwise written Tiitsch, 
Teutonicus, Theotonicus, Theotiscus, and endless other ways. He 
can hardly think that I have never opened a modern German 
book : I can hardly think that he has never opened an English 
book of the sixteenth or seventeenth century. If he has opened 
any such book in which matters of this kind are likely to be 
touched upon, he must surely know that the words Dutch and 
Dutchman were then used in a very wide sense. A ‘ Dutch- 
man” might be a native of Holland; he might be a native of 
Bavaria. And the division into High Dutch and Low Dutch, 
or Vether Dutch, was then perfectly well understood. 
I do not know what Mr. Huxley’s objection is. I use the word 
as one ready made, as more convenient than the Latin for Zeu- 
tonic, and as more easily admitting the addition of the qualifying 
syllables High and Low. I should not use Deutsch in this sense 
for two reasons. First, it is not an English form, and I should 
no more, in writing English, say that certain people talked 
“Deutsch,” than I should say that they talked “* Frangais.” 
Secondly, the word Deutsch (like the word German) would to 
most people convey the idea of one particular Teutonic tongue, 
while I am probably speaking of Teutonic tongues in general. 
Mr. Huxley’s question, ‘* Did the Germans of Caesar and Tacitus 
speak Deutsch ?” may mean either Did they speak a Teutonic 
tongue of any kind?” or ‘* Did they speak the particular Teu- 
tonic tongue which to most peoples’ minds would be suggested 
by the words German or Deutsch, namely, the Liigh-Dutch ?” 
Which Mr. Huxley means I do not know. For my own part I 
believe that they spoke Dutch or Teutonic, but Zow Dutch and 
not A/igh. Epwarp A. FREEMAN 
Somerleaze, Wells, March 21 
The American Eclipse 
WILL you grant me space for a few words on my spectro- 
scopic observations of the American eclipse, and what seem to 
me the inferences to be drawn from them? I make the re- 
quest the more freely because I have met from time to time allusions 
to them in your journal, and remarks, some of which seem 
to require my notice, if only to express my appreciation of 
