362 
expect to sce the same rainbow directly and by reflection. It is 
also reasonable to suppose that, as a rainbow is often seen from 
one place and not from another, a rainbow may often be seen 
directly and not reflected, or vice versé. The reference to the 
necessary condition of parallelism shows that it is something 
more than these obvious deductions from the laws of reflection 
to which Prof. Tyndall wishes to draw attention in the passage 
mentioned. Until I tried the experiment described below, I 
imagined him to mean that there was something about the direc- 
tion or arrangement of the rays of light producing a rainbow, 
which prevented their forming a rainbow or anything like one, 
after reflection from the surface of still water. It is not always 
easy to arrange so as to have a rainbow and a still lake to ex- 
periment upon. I managed, however, to get satisfactory sub- 
stitutes in the spray bow at the falls of the Rhine near this, and 
a small pool of water. I was greatly disappointed on looking 
into my pool, to see reflected not only the scenery of the falls, 
but also a very fine spray bow. 
What then can Prof, Tyndall mean? How is this peculiarity 
of rainbows to be observed? I have tried it in the only way of 
which I could think, but am now inclined to believe that I must 
have mistaken Prof. Tyndall’s meaning. 
Schaffhausen, Aug. 23 ne Ss 
The Origin of Nerve Force 
Ong at least of the ‘‘ obvious difficulties” which your corre- 
spondent, Mr. Henry R. Proctor, finds in my hypothesis as to 
the origin of Nerve Force, would scarcely have existed if he had 
directed his attention to a sentence in my article (NATURE, 
July 31), which runs thus: ‘‘In what are termed hot-blooded 
animals, that is, in mammals and birds, the difference of tempe- 
rature between the surface and the interior is considerable under 
all natural circumstances, and in them there is a regulating action 
of the skin by which they maintain a uniform internal tempera- 
ture, always hotter than the surface, whatever that of the external 
medium may be.” The correctness of this proposition as regards 
the human being“is now a physiological fact, as many observers 
from different starting points have arrived at the same conclusion ; 
among others, my proof of it has appeared in the “ Journal of 
Anatomy and Physiology” (vol. vi. November 1871). When the 
temperature of the atmosphere is above 70° F. the amount of 
perspiration is always proportionate to the temperature, and is 
sufficient to maintain the depths of the body at 98° or so. Below 
70° the same condition results from the influence of cold on the 
cutaneous vessels, they contracting in proportion to the degree 
of cold, and so modifying the radiating and conducting power of 
the body surfaoa. There is never therefore any reversal of the 
current, or a temperature at whichit is sz/. 
Your correspondent’s third paragraph contains an assumption, 
as great and not so reasonable as my own. Why should we 
have to assume that the body has to be kept at a constant tem- 
perature of 98° or so? There is no @ Prior? reason in its favour. 
It may be said that the chemical changes which occur, being de- 
pendent on the properties of albumen, fibrin, &c., could not 
be continued under other circumstances. That, however, is 
only a shifting of the ground of argument, for it is much more 
reasonable to suppose that the properties of the animal tissues 
are the result and not the cause of the conditions under which 
they have been brought into existence. 
I may mention that the physiological phenomena attending 
the immersion of the body in air and water of different tempe- 
ratures are of quitea different character ; they are scarcely com- 
parable, and can be shown not to depend to any extent on the 
different conducting powers of the media, or their different spe- 
cific heats. Immersion of the nude body in air of 30° is not 
rapidly fatal, even if the temperature is not kept up by violent 
exercise ; and I cannot understand “immersion in water at 
30°. ” 
If the comparative coldness of the brain were the effect of 
absorption of heat in the building up of its elaborate texture, 
we should expect to find a similar condition in the muscles, 
which are also of very complicate construction. Such, however, 
is not the case, and therefore another explanation has to be 
found, which my hypothesis supplies. 
Aug. 26 A. H. Garrop 
The Flight of Birds 
I HAVE just read with great interest, in NATURE of Aug. 21, 
Capt. J. Herschel’s account (elicited by Mr. Guthrie’s letter, 
NATURE 
[ Sepz. 4, I 873 
vol. viii. p. 86) of his ocular and telescopic observations or 
Indian kites at rest in mid-air, and I am tempted to offer an 
explanation which occurs to me of the way in which that 
aérial balance may be maintained. 
If there was no quiver of the wings perceptible ‘‘ at an appa- 
rent distance of ten or twelve feet,”—if the very tips of the 
wings ‘‘ looked as steady as those of a stuffed specimen,”—then 
certainly the theory of self-support by muscular action must be 
abandoned, and the problem is reduced to one in which we have 
only to consider the weight and shape of the bird with outspread 
wings and the velocity and direction of the wind. 
If the direction of the wind is slanting upwards with mode- 
rate velocity, it is conceivable that a bird, facing the wind, with 
outspread wings in a plane inclined between the horizontal 
and the direction of the wind, might remain at rest, from the 
following considerations :— 
If the air were at rest, the bird, with the plane of its wings 
inclined a little downwards and forwards, would not fall yer- 
tically, but would slide obliquely forwards down the air, like a 
returning boomerang, or an inclined sheet of paper let iy 
and would reach the earth at some point far from the verti 
But suppose, instead of the air being at rest, there were a slant 
upward current of air meeting and balancing the slant fall of the 
bird : then the bird would remain motionless in mid-air. ; 
Capt. Herschel rejects (perhaps too hastily) the notion of 
‘*slants of wind,” and asks ‘‘ what becomes of the horizontal 
force” of the wind. Surely its effect would be to balance 
the horizontally resolved portion of the bird’s slant fall, just as 
the vertically resolved portion of the slant current of wind would 
balance the vertically resolved portion of the slant fall. 
Different degrees of inclination and force of the wind might 
be met (within limits) by different degrees of slope and spread 
of the wings. 
I must confess this is only theory. We want more obser- 
vations, as keen and careful as Capt. Herschel’s, to ascertain 
the force and direction of the wind attending this arrest of 
motion in mid-air. Slant currents are common enough on a 
small scale among house-walls, and ona larger scale we may 
see how the wind pounces down on a land-locked water, or 
presses up a mountain side. In a steady wind, the shapes of 
hill and valley must cause certain regular currents variously 
inclined to the horizontal, and some of these, I suppose, the 
eagles find and use. On the lee side of a hill (as in the case 
given by Captain Herschel) there would be a current rising 
trom the eddy to join the main course of the wind. ‘The con- 
ditions described by Mr. Guthrie were just such as would throw 
the wind into upward slanting currents. 
We should want a well-balanced weather-cock with a double 
vane (one plate in a Aorisonéal, the other in a vertical plane), to 
tell the vertical as well as the horizontal deviation of the wind. 
Dacre Park, Lee, S.E., Aug. 24 HUBERT AIRY 
Mallet-Palmieri’s ‘‘ Vesuvius ” 
My absence in Spain during the months of March and April 
prevented my having seen NATURE for the 20th March, and left 
me until a few days since in blissful ignorance that it contained a 
lengthy critique by Mr. Mallet on my review (NATuRE, Feb. 6) 
of his translation of ‘* Palmieri’s ‘‘ Incendio Vesuviano.” This 
accounts for my silence, as, had it not been the case, a reply 
from me would certainly have appeared at the time. 
For, being “‘ the reviewer reviewed,” I suppose I am indebted 
to my habit of not taking advantage of a reviewer's privilege, 
but of signing my name in full, since I do not find that Mr. 
Mallet vouchsafed a reply to any other review of his book, not 
even to that contained in the Geological Magazine for March, 
which, as the organ of British Geological opinion, might be ex- 
pected to have the preference over mine, even if its reviewer had 
not incurred special claims on Mr. Mallet’s attention, by having 
handled his production in a vastly less tender manner than I had 
done. 
In comparing the two translations of Palmieri’s little pamphlet, 
I give preference to that in German by the eminent mineral 
chemist Rammelsberg, if for no other reasons, for its cheapness, 
and because the translator puts forth the work of the Italian 
professor entirely on its own merits as one which did not require 
to be heralded by any elaborate preface to make it take with the 
public, and also because it seems somewhat unfair to see the 
worthy Professur’s excellent observations made a y-hicle for in- 
| troducing the public to what, although entitled ‘‘an introduc. 
