266 
is not entirely satisfactory. . . . The modern affinities of the 
numerous leaves of the higher types of dicotyledonous trees 
found in it, present a strong objection to the adoption of the 
conclusion that it may belong to a lower horizon than the Upper 
Greensand of British geologists; while its position directly 
below beds almost beyond doubt representing the Lower or 
Gray Chalk, precludes its reference to any higher stratigraphical 
position, Consequently, we have long regarded it as most 
probably representing in part, if not the whole, the Upper 
Greensand. ...” As the fossils above mentioned belong to 
the Museum of Comparative Zoology, I hope te be able to give 
more detailed information upon these galls and mines. 
Cambridge, Mass., January 3 H. A. HAGEN 
On Combining Colour-Disks 
THE paper of Lord Rayleigh in NATURE (vol. xxv. p. 64) 
describing experiments on colour, gives near the close a method 
of observing the image of colour-disks seen through an inverting 
prism in rapid rotation, while the disks were at rest. This recalls 
to mind a method somewhat similar that I have tried, that will 
sometimes be found convenient as well as simple: Here the 
image of the stationary disks is formed in a plane mirror slightly 
inclined to the axis around which it rotates; by properly pro- 
portioning the angle of inclination, the distances from the 
mirror to the eye and disks, and the sizes of the mirror and disk, 
it is obvious that a good combination of the colours may be 
effected, while the adjustment of colours is easily effected with- 
out stopping the rotation. If, as with my instrument, the clock- 
work is not heavy enough to give easily the desired speed when 
the disks are mounted on it, a much higher speed can be ob- 
tained with the light mirror: indeed, the mirror might be 
attached to the end of a wire resting on two supports, and 
rotated by unwiuding a string, aud thus colour combinations 
could be simply effected, and with cheap apparatus. Of course 
here, as with the inverting prism, the line of vision is inconve- 
niently limited ; but with both methods the uncertainty arising 
from unequal illumination of different parts of the disk may be 
detected by giving to the disk a slow rotation on its own plane, 
CHARLES K, WEAD 
University of Michigan, Ann Arbor, U.S.A., 
December 31, 1881 
Sound-Producing Ants 
WITH reference to the question whether ants produce sounds 
which are of such a pitch as to be inaudible to the human ear, 
I should like to make a suggestion which occurs to me, but 
which I have no means of carrying out practically. It is a well- 
known acoustical fact that two notes of high pitch sounding 
together produce a third whose vibrational number is the differ- 
ence of the vibrational numbers of the two primary notes. If 
now we suppose a vibration at the rate of (say) 60,000 per 
second, another at the rate of 38,000 per second would give a 
difference note of 22,000 per second, which would be well 
within the range of audibility. If then we send up a note 
beyond the extreme limit of audibility, we shall be able to detect 
the presence of vibrations which exceed that of the note sent up 
by the highest number of vibrations of audible sound. It would 
be interesting to know if this has been attempted, and if the 
microphone can be applied to assist in the investigations. 
Hirwain, Aberdare, January 10 D. M, Lewis 
Nepotism ? 
PLEASE, Mr. Editor, is a pet baboon (NATURE, vol. xxv. p. 
217) more interesting than either a pet sparrow or a pet canary 
bird ? Don’t give rise to the suspicion that there is any nepotism 
in the affair ! Joun H, VAN LENNEP 
Zeist, January 10 
INDIAN Fossits.—Mr. J. W. Oliver informs us that at the 
Birmingham and Midland Institute there is a small eollection of 
Siwalik fossils which, he understands, were sent some years ago 
from the British Museum. Prof. Prestwich writes that there is 
a very large and fine collection in the Oxford University 
Museum, presented by Dr, Falconer and Sir Proby Cantling. 
Prof. Prestwich will be happy to give Mr. Lydekker every 
facility for the examination of the speciment. 
NATURE 
[ Fan. 19, 1882 
COMPRESSED AIR UPON TRAMWAYS 
FEY persons unconnected with the practical working 
of the companies are aware of the great amount of 
time, labour, and money which have been devoted to the 
substitution of mechanical for horse-power upon tram- 
ways both in this country and abroad. The principal 
incentive to this exertion has been the large margin of 
saving which has presented itself in the light of a pre- 
mium to inventors and capitalists. Motives of humanity 
towards the horses have also had considerable influence, 
especially with Parliament, and have contributed in no 
small degree to the legislative sanctions which have been 
obtained not only by particular companies, but by the 
tramway interest in general. In no case however that 
the writer is aware of, have the tramway companies 
themselves made any material contributions towards the 
solution of the problems involved. When the story of the 
subject comes to be written it will be found full of 
arguments in favour of the principle that the monopoly 
granted to inventors by the patent laws is nothing more 
than a clumsy method of spurring them to exertion, and 
of providing a remuneration for success which never 
covers the aggregate losses of failure by which the whole 
community have been indirectly benefited. 
The fact of the horse-tramway companies having re- 
fused to assist inventors with money is fully accounted 
for and rendered excusable not only because they have 
no funds placed at their disposal by their articles of 
association for such a purpose, but also because the 
investment would have been far too speculative to have 
been sanctioned by the shareholders. Where the com- 
panies appear to the writer to have been at fault is that 
while the margin of saving as between a successful inven- 
tion and horse-traction is admitted to be enormous, be- 
cause the invention could hardly be said to be successful 
unless the margin was a large one, they have never 
admitted either individually or collectively that some sub- 
stantial share of the saving should be the reward of the 
successful inventor. The writer has no hesitation in saying 
that if the leading companies had put the issue clearly before 
the inventive capacity of the engineering profession in the 
shape of an offer of say 30 per cent. of the actual saving 
in the shape of royalty to the inventor that the problem 
would have been solved at least six years ago. ‘The far- 
stretching results of such a revolution, even within the 
comparatively confined area of the tramway interest, would 
be incalculable. Not to speak of the emancipation of the 
horses, the employment of capital in channels so con- 
sistent with the spirit of the age and the genius of the 
country as the manufacture of machinery would have 
; econoniic results affecting the welfare of whole classes of 
the community, and the impetus given to the intramural 
locomotion of our large cities would go far to overcome 
the pressure of difficulties affecting the housing of the 
poor, which contribute more to the unrest of the people 
and the propagation of socialistic ideas than the wealthier 
classes are aware of. The policy of the tramway com- 
panies, however, appears to have assumed the character 
of a fixed determination to give nothing in return for the 
advantages which would accrue to them from the adoption 
of a successful mechanical substitute for horses. So long 
as they maintain this attitude the problem is likely to 
remain unsolved. Licensees of inventors have followed 
their example, and at least one case is known to the 
writer in which a gross breach of agreement has debarred 
the adoption of an invention which is notoriously effi- 
cient. Time no doubt will expose the guilty parties, and 
their names, instead of being honourably associated with 
the advance and improvement of mechanical science, 
will be handed down to posterity with the contempt which 
they deserve. 
A description and illustration have already been given 
in these pages of a system of tramway traction by means 
of electricity, and this is no doubt safe in the hands of 
