SCIENCE. 
23 
MICROSCOPY. 
Dr. Henri Van Heurck, the well-known director of the 
Botanical Gardens at Antwerp, Belgium, proposes to issue 
a “ Synopsis of the Belgian Diatoms,” in a series of six 
numbers, each one to contain about a dozen plates. The 
division of plates will be such that the great groups of dia- 
toms will be comprehended each in two parts, as follows : 
I and II, Raphidece : Amphorece , Cymbellece , Naviculece , Gom- 
phonemece , etc. ; III and IV, Pseudo- Raphidece: Epithemiece, 
Synedrece , Sutirellece , Nitzschiece , etc. ; V and VI, Crypto-Ra- 
phidece : Melosirece , Coscinodisei, etc. This arrangement is 
that proposed by Prof.Hamilton L. Smith in the general syn- 
opsis of the Diatomacese as inserted in the “ Traite du Micro- 
scope ”(3e Edition Bruxelles, 1878), de Mr. le Dr. Henri Van 
Heurck. 
The price of each plate, accompanied by its description, 
is 75 centimes (15 cents), to subscribers ; to non-subscribers, 
after publication, the price will be one franc (20 cents), per 
plate. The numbers will appear at intervals of three to four 
months. The text will be published after completion of the 
plates. It will embrace a description of all the forms hith- 
erto found, or likely to be found in Belgium, indicating 
localities, etc., and with synoptical tables for determination, 
etc., etc. The price of this volume is fixed at 7 francs. 
Dr. Van Heurck has sent to me a limited number of spec- 
imen plates and the prospectus of above work, which I will 
be pleased to send to any one taking special interest in the 
diatoms, and intending to subscribe. 
The name of Dr. Van Heurck is a sufficient pledge that this 
“synopsis” will be issued strictly according to the prospectus, 
as announced above ; and it cannot fail to be very accept- 
able to every student of these beautiful and wonderful 
microscopical plants. The “ British Diatomacese ” is now 
almost beyond reach, and nothing that I know of will so 
nearly supply its place as Dr. Van Huerck’s proposed Syn- 
opsis. Besides containing probably all the species described 
in the “British Diatomacese,” there will be many not in- 
cluded in that, and embracing by far the greater number of 
forms found in our own country. 
It will give me pleasure to communicate any further in- 
formation. Address, “ Monsieur le Dr. Henri Van Heurck, 
Directeur du Jardin Botanique, Rue de la Sante, 8, Anvers, 
Belgique.” 
I will only add that the plates are heliographic reproduc- 
tions of enlarged drawings made by Dr. Van Huerck, or by 
M. Grunner, and that M. Deloyne of Brussels proposes to 
issue a series of diatom preparations, in boxes containing 
twenty-five slides each, similar to those of my own “ Species 
Typicse,” and in accordance with the synopsis of M. H. Van 
Heurck. H. L. Smith. 
Hobart College, July 1, 1880. 
NATURAL HISTORY. 
Mr. Proctor remarks that among the problems with which 
science has not as yet succeeded in dealing satisfactorily is that 
of the flight of birds, and especially the flight of those birds 
which float for long periods of time without any apparent 
movement of their wings. During my voyage from San 
Francisco to Honolulu (which latter place, by the way, I 
have not reached at the moment of writing to the Newcastle 
Weekly Chronicle, 2.20 P. M., April 17th, ship time — lat. 
about 26° 35' and longitude about 145 0 west, so that Green- 
wich time is about midnight, April 17th) I have noted with 
much interest the flight of the birds — the sailors call them 
mollyhawks — which follow the ship apparently without ceas- 
ing, except for an occasional short rest on the water. It is 
certain that for many minutes together — in some cases I 
should say for fully ten minutes — these birds do not use their 
wings except to guide their movements and to sustain their 
bodies in the same sense that a parachute sustains a weight 
suspended to it — that is, they do not make active use of 
their wings, though of course a certain degree of muscular 
exertion must be involved in the mere sustentation of the 
body. I have seen nothing yet to confirm the statement I have 
often heard made, that these birds, albatrosses, and others, 
will float about, sustaining their bodies in this, as it were, 
passive manner during much longer periods of time, as an 
hour or so. I should be inclined to doubt whether a bird 
could be, or has been, steadily watched even for half an hour. 
But if they do, the problem is not altered in character, but only 
in degree. N ow it is manifest, in the first place, that the flight 
of a bird is not — as some who reject all attempts at explana- 
tion, would seem to implj' — a miraculous phenomenon, but 
one purely dependent on ordinary mechanical laws. The 
muscular power shown by birds may be, and indeed is, very 
marvellous. The perfect adjustment of all their move- 
ments to obtain the greatest possible effect from every 
muscular effort, might probably be shown to be equally so, 
if we were able to analyze each movement as made, instead 
of being foiled as we are by the exceeding rapidity of a 
bird’s evolutions. And, again, it is possible that the sus- 
taining power of the air on bodies of particular form travel- 
ing swiftly through it maybe much greater or very different in 
character from what has been hitherto supposed. But it is 
quite certain that the flight of birds depends on ordinary 
laws, however difficult it may be to explain it by their means. 
It may be a step towards the solution of the problem to con- 
sider what attempted explanations must at any rate be re- 
jected. Amongst these is one which has been often ad- 
vanced, and which seems to have a singular attraction for 
unscientific persons — the theory, namely, that the bones and 
quills of a bird are filled with some light gas, floating the 
bird in the same way that balloons are raised by the hydro- 
gen gas within the silk. Those who hold this theory seem 
to imagine that hydrogen possesses some lifting power, as 
though the gas of itself sought to rise upwards from the 
earth. In reality, of course, hydrogen obeys the law of 
gravity and is drawn downwards, and not upwards. It 
rises much as the least heavily loaded scale of a balance 
rises — not because its own tendency is upwards, but be- 
cause something else has a stronger tendency downwards. 
If a balloon instead of being filled with hydrogen were 
absolutely empty, and could yet retain its shape against the 
pressure of the surrounding air, it would rise more quickly 
than when filled with hydrogen, for the simple reason that 
it would be relieved of the weight of the hydrogen itself, 
which, though much smaller than that of an equal volume of 
air, still counts for something. Similarly, if the quills 
and hollow bones of the bird absolutely empty — no 
air nor the lightest gas being present in them — the 
lifting power resulting from this condition of things 
would be the greatest possible under the circum- 
stances. A yet greater, in fact a very much greater 
lifting power would result if the whole body of the 
bird were hollow and vacuous. But how ineffective even this 
lifting power would be to raise the actual weight of the bird 
may be seen from the following simple considerations : — 
The specific gravity of a bird is certainly not less than a 
third that of water, as may be shown at once by observing 
how much of a bird’s body is under water when the bird is 
floating. We may then safely assume that a bird’s specific 
gravity is equal to 200 times the specific gravity of air. The 
difference then between the weight of the air displaced by 
a bird’s body and the no-weight at all of an equal volume 
absolutely void, is only i-200th part of the actual weight of 
the bird’s body. This is the whole effective lifting force 
even in the perfectly imaginary case in which the entire vol- 
ume of the bird is supposed to be available for this kind of 
support. The remaining i99-200ths, or practically the 
whole weight of the body, is left unsupported in this way, 
and some other explanation of the observed fact that it is 
supported remains to be sought for. I believe the true ex- 
planation is to be found in the enormous propulsive power 
of a bird’s wings, combined with the perfect balance which the 
bird is able to maintain, with such changes only as may 
be rendered necessary by the changing direction of his 
motion. Of course I am aware that gravity acts with 
equal efficiency on a body traveling swiftly as on a 
body at rest. A cannon-ball allowed to fall from the 
mouth of a cannon reaches the earth no more quickly than 
one fired horizontally from the cannon’s mouth. But I be- 
lieve that a flat body travelling swiftly in a horizontal di- 
rection with its plane horizontal, sinks far more slowly 
earthwards than one of a similar shape which is not advan- 
cing or is only advancing slowly. The difference may be 
compared to that which would be noted between the fall of a 
flat stone on the surface of water when the stone is allowed 
simply to drop, and when it has been propelled horizon- 
