506 
NATURE 
[ Oct. 26, 1871 

his difficulties would vanish, and he would already know the 
proposition. 
All that I contend for is, that the new book on geometry 
ought to be capable of such usage. If it contains little more 
than the chief steps of the solutions, and those disguised (to the 
unpractised and tottering mind) under symbols, it will not satisfy 
the want now felt. A FATHER 
The Beef Tapeworm 
As an entozoologist and correspondent of the Academy of 
Natural Sciences of Philadelphia, I request permission to correct 
an error recorded in the report of the Academy as given in your 
columns (at p. 500) this week. Ir. Leidy is represented as 
having stated that ‘*the minue acetabular pit or fovea at the 
summit of the head [of Zwzéa mediocanellata] is not mentioned 
by Kuchenmeister and subsequent observers as a character of that 
species.” I beg to remark that I both figured and described 
this supplementary sucker-like structure in the first edition of my 
small work on ‘* Tapeworms,” published in 1866 (p. 33 e seg ). 
At least two other observers have figured and described this 
central depression, not only in the aduit but also in the measle 
or cysticercal stage of the worm. Lven Bremser recognised it, 
but his description was for a time overlooked. 
84, Wimpole Street, London, Oct. 21 T.S. CoBBpoLp 
Winter Fertilisation 
In the first number of NaTURE, (for Nov. 4, 1869,) I ventured 
on a hypothesis, jound:d on a series of observations, that plants 
which flower in the winter have their organs of reproduction 
specially arranged to promote self-fertilisation, The following 
fact, which has just come under my notice, appears to confirm 
this theory. Planis belonging to the order Caryophyllacee are, 
as a rule, strongly protandrvus (see my paper in the Journal of 
Botany for October 1870), the anthers discharging their pollen 
at so long an interval before the maturing of the stigma as to 
render cross-fertilisation almost inevitable. The other day, 
Oct. 21, I came across a late flowering patch of Sve//avia aguatica 
Scop., in which the anthers were discha ging their pollen simul- 
taneously with the maturing of the stigmas, each of the five styles 
being curled in a sirgular manner round one of the stamens, so 
as to bring the stigmatic surface in actual contact with the dehi-cing 
anther. This occurred in several flowers that were just opening, 
and there was abundance of seminiferous capsules on the plants. 
ALFRED W. BENNETT 
Velocity of Sound in Coal 
Your correspondent will find in Prof. Tyndall’s beautiful 
work on ‘‘ Sound ” the data required for the exact determination 
of iis velocity in different media. I believe that in coal it will 
be found to be between six and seven times that in air, or about 
7,000 feet per second. 
If Mr. D. Joseph places his ear against the solid coal of the 
**rib” or side of the ‘‘ heading” or gallery, at a distance of some 
twenty to thirty yards from a collier at work, he wiil hear two 
sounds for each blow of the workman’s pick or mandril—the first 
being transmitted through the coal, the second more slo«ly 
through the air, the impression being almost irresistible that two 
persons are at work. 
This is probably the origin of the legend, common in more 
than one coal district, of a collier who always worked alone, did 
more work than his fellows, and whose diabolical assistant was 
often heard but not seen. Cay: 
Changes in the Habits of Animals 
Your correspondent Mr. Potis in the last number of NATURE 
furnishes us with a few interesting facts regarding the Aca. In 
a peper which I read about three years ago to the Dumfries 
Natural History Society, entitled “ The Influence of the Human 
Period on the Sagacity of Animals,” and subsequently in a letter 
published in Narurr, vol. i, on the ‘‘ Mental Progress of 
Animals,” I endeavoured to show from general considerations, 
and from the few facts which we possessed on this subject, that 
the habits and instincts of animals were not so fixed and definite 
as might be supposed. The general principle for which I con- 
tended was that whether we considered the globe to have received 

its human inhabitants according to the laws of evolution, or in 
some miraculous manner, the arrival of the human race pro- 
duced great modifications and changes of surrounding circum- 
stances. These changes were in the direction of increasing the 
fertility of all vegetable productiuns capable of sustaining life, 
and at the same time securing their use entirely for the human 
family. Hence arose, in the vicinity of man, two new factors ; 
the superior attraction of better food for all kinds of animals, 
and at the same time the extinction of such animals whose greed 
was not overruled by sufficient wariness or cunning to become 
successful thieves. Hence a probable gradual increase in these 
qualities in the animals maintaining themselves against man. 
Since my attention was drawn to this subject, we have had some 
interesting observations on modifications of swallow’s nests by 
Pouchet, and a discussion as to the validily of his conclusions by 
Noulet,and now Ihave read with pleasure Mr. Potts’s observations. 
Most likely the progress of development in the carnivorous habits 
of the Kea will meet with a check now that shepherds are alive 
to its depredations ; but without the influence of the human 
period we can scarcely suppose that such development would have 
begun, I recollect a case of change of habits in weasels. They 
multiplied so thickly in a parish in the south of Dumfriesshire 
that some hungry philosopher among them took the initiative in 
sucking the blood from the cattle. Suspicion having been 
aroused, the fact was proved, but its discovery was fatal to the 
weasels, for the whole country-side arose against them, and all 
but extirpared them in that quarter. It is very interesting to 
observe what modifications are being produced in the habits of 
various species of sca-gulls since Glasgow, by its enormous in- 
crease of commerce, has wrought great changes »n the River 
Clyde, filling it with all kinds of garbage. The conditions of 
existence having been tavourable, the guil issteadily pa-sing more 
and more time inland ; ascending tributaries of the Clyde, and 
alighting in flocks on fields that used to have him very seldom. 
A pew amusement within my own recollection has been 
afforded the river passengers during the summer months in feed- 
ing these sea mews, &c., by throwing overboard food to them, 
and their increased tameness and boldness of approach in follow- 
ing the river steamers within the last thirty years have been fre- 
quenuy commented on, J. SHaw 
Oct. 23 
A Plane’s Aspect 
Mr. LAuGHTON has hit the nail on the head. ‘‘ Aspect” is 
exactly the word wanted. The aspect of a plane is the direction 
of its normal ; and ‘“‘ parallel planes are defined as those which 
have the same aspect.” Two aspects determine one direction, 
and two directions determine one aspect. Mr, Laughton deserves 
the ‘hanks of geometers for suggesting so good a word. 
Rugby, Oct. 23 J. M. WILson 
THE words ‘‘aspect”’ and ‘‘slope” have already a use in re- 
lation to the position of planes. They indicate two elements 
which /ogether fix the position. Neither of them, taken alone, 
can indicate the position of a plane, unless a new and artificial 
meaning be assigned to one or other. Thus if I speak of the 
‘aspect’ of one of the faces of a roof as southerly, I have done 
something but not all that is necessary, towards describing the 
position of that face; if I add further that the ** slope” is 30° T 
have definitely assigned the position. Again if I speak of the 
“*slope” of Saturn’s rings as 28° (the plane of reference being 
ecliptic), I have done something towards the description of their 
position ; if I add further that their ‘‘aspect” is toward such 
and such a degree of the sign Gemini, I fully assign their posi- 
tion in space. And so on. 
In the preceding sentences I have used the words ‘‘slope” and 
‘‘aspect ” as they are already understood. I apprehend that I 
have also used the word ‘ position” as it is already understood, 
and that no other word could properly be used in the same sense 
in descriptive writing. I can see no reason why “position” 
should be dismissed from the position it has so long occupied, 
nor why ‘‘aspect” and ‘‘slope” should be regarded in a new 
and unfamiliar aspect. 
It chances that I have long since had occasion to consider the 
question suggested Jast month by Mr. Wilson. In each of 
twelve books which I have written during the past six years, I 
have had repeated occasions to c nsider the slope and aspect, 
that is, the ‘* position” of many important astronomical planes. 
