a 
198 
NATURE 
[December 31, 1896 
height by this method is too low. The average determined from 
reflected lights ($45 metres) is probably most nearly correct, 
I think it is apparent that the observations with theodolites 
and photogrammeters at the international cloud-stations should 
be supplemented by other methods, if correct averages are to 
be obtained, and if clouds which cover the sky with a uniform 
veil are to be measured at all. Small balloons turned loose and 
followed with theodolites, suggested by Kremser, is a good 
method in such cases. H. HELM ChaytTon. 
Blue Hill Metec srological Observatory, December 7. 
Radiography. 
YOUR correspondent, Mr. G. M. Lowe, asks for information 
as to the best methods of working direct on to sensitised paper 
to save the time and expenses involved in taking glass nega- 
tives. ‘* Nikko” paper, as supplied by the Eastman Company, 
is a good substitute for glass plates, and the results on it are 
much superior to the smooth bromide papers. _Eikonogen is 
a suitable developer—say five or six ounces of water to the 
contents of an eikonogen cartridge, and fix in clean hypo 
solution. Of course, to show the palma-surface of the hand 
when using a glass plate, the film side is wf and the palm 
down ; but in using paper direct, the film should be down, and 
the X-rays, therefore, pass through the paper before reaching 
the sensitive surface. 
Radiographs made direct on paper are xegatzves, the bones 
being shown white. It has been stated that this is the correct 
way to show the bones, but it is quite a mistake. Bones are 
white by reflected light; by transmitted light they are black, 
more or less, and if X-rays are light rays, then the light is 
transmitted, and radiographs ought to be as usually shown 
printed from a glass negative; but for surgical purposes, for 
such as foreign bodies in the hand, negative or positive makes 
little or no difference. By the direct ‘‘ Nikko” paper method 
the exposure must be longer, but to locate a needle in the hand 
thirty to forty seconds is sufficient. Two, three, or up to a 
dozen sheets of *‘ Nikko” paper may be exposed at one time. 
Between the first and second sheet very little difference in ex- 
posure will be noticed, but between the first and, say, the 
twelfth the difference will be considerable. To extend this 
difference when only a few sheets are used, insert a piece of 
suitable black paper between each. W. I. Cuiapwick. 
The Heating of Anodes in X-ray Tubes. 
I sHALL be much obliged if any of your readers who work 
with the X-rays will give me their experience with the 10-inch 
coils. I have one by Apps, which is excellent in every way ; but 
whether I take from it a 2-inch or a 10-inch spark, the anode of 
the tube invariably becomes red or white hot within a few 
seconds, 
The tubes are by leading makers, and exhausted for 8 or ro- 
inch sparks; but, as I have said, even a 2-inch spark makes the 
anodes red hot. 
On the other hand a German coil I have, does not perceptibly 
heat the anode of the tube even when I use a 5-inch spark. 
Is this the experience of others ; and why should a 24-inch 
spark from one coil make the anode red hot immediately, when 
a 5-inch spark from another coil does not do so ? 
This difficulty at present prevents me employing the Apps ro- 
inch coil at all for X-ray work. WALTER CHAMBERLAIN. 
Harborne Hall, near Birmingham, December 19. 
Units of Force. 
IN your issue of December 10, Prof. O. J. Lodge makes 
several curious statements. 
He speaks of ‘‘ inertia multiplied by the square of a velocity.” 
He might as well speak of ‘‘shapelessness multiplied by the 
cube of a length.” Inertia is a word best left unused, but 
usually means a property of what is called matter—like white- 
ness, hardness, inextensibility. 
He also speaks of natural formule “independent of every 
system of units that can be devised,” and, though he only gives 
one formula, implies that every mathematical relation can be 
expressed in a similar manner. Will he be so good as to give 
a formula connecting the weight, volume, and specific gravity of 
a body which is ‘‘ independent of every system of units ” ? 
As to the poundal, the objection to it is that no one uses it in 
NO. 1418, VOL. 55 | 
. to lower it to its summer position. 
actual work. 
sufficient one. 
As to teaching elementary mechanics, I am convinced that we 
should avoid ‘*mass” as much as possible. When dealing 
with a particle, express Newton’s Second Law by the formula 
P/Q = //a, where P and Q are the forces producing accelerations 
There may be other objections, but that is a 
f, a, respectively. This will usually take the form P/W = ile: 
Then you may use any unit of force you choose, and the energy 
formula becomes P x s = W“_ ; which may be in inch tons, 
2s 
foot pounds, or what you please. Is it too much to hope that 
the poundal may be shortly relegated, even in text-books, to 
that place, wherever it is, where grades are employed for 
measuring angles ? C. S. JACKSON. 
R,M. Academy, Woolwich. 
The Distance of the Visible Horizon. 
Has not Prof. Lodge in his enthusiasm, which I fully share, 
for an absolute system of measurement rather overstepped the 
mark when in the equation 2 R 4=d? for the distance of the 
visible horizon, he says that ‘‘ 4 is not the number of feet, or of 
metres, or anything else, it is the actual height; @ is not the 
number of miles or of inches to the horizon, but it is the distance 
itself; and similarly 2 Ris the diameter of the earth, and not any 
numerical specification of that diameter (see NATURE, vol. lv. 
page 125). Surely the equation as written is an algebraical 
equation, and, as such, the symbols it contains express numbers 
and not things. The multiplication as he implies of one length 
(2 R) by another length (4), is abhorrent to the mind of ‘‘the 
Cambridge mathematician.” The superiority of the formula 
over the mutilated apology for it which Prof. Lodge quotes, lies 
in the fact that the equation is true in terms of any conceivable 
unit of length in which the three lengths involved in it are 
measured. Iam of course aware that the particular formula 
given may be regarded as an abbreviated statement of the 
approximate geometrical proposition that the rectangle contained 
by the diameter of the earth and the height of the observer above 
its surface equals the square ona line equal to the distance of 
the visible horizon, in which case, of course, Prof. Lodge's 
description of the symbols would be accurately true; but I do 
not think that the formula with this interpretation really illustrates 
his meaning. 
I wish to associate myself with Prof. Lodge in his condemna- 
tion, for educational purposes, of all formulz of the engineer's: 
pocket-book type, should it unfortunately happen, that they 
gain a footing on the scientific side of school instruction it will 
do much to justify the slur, still too often cast, on science 
teaching at schools and at the universities, that it is not education. 
This must be my apology to Prof. Lodge for thus emphasising 
a mere dapszus calamiz. L. CUMMING. 
Rugby, December 12. 
Position of Boughs in Summer and Winter. 
THE following measurements may perhaps be of interest. 
They have been made witha view to ascertaining how much the 
weight of leaves and fruit depressed the branches of a tree. The 
first measurements were taken on August 3, the second on 
December 14, 1896 :— 
Leight from Ground tn inches. 
Mulberry tree— August 3. December 14. 
Lowest twig ao on O in. 31 in. 
Higher branch 59 in. 72 in. 
Another branch 20 in. 39 in. 
Walnut tree— : : 
Lowest twig 15 in. 34 in. 
Higher branch 60 in. 76 in. 
In the case of the first branch of the mulberry tree, it was 
found in December that a weight of 35 pounds was not sufficient 
AGNES FRy. 
Failand, near Bristol, December 15. 
The Cultivation of Woad. 
WITH reference to the letter of Rosa M. Barrett, in NATURE 
of November 26, p. 79, I formerly lived for many years, 
and my father before me, in the part of Somerset to which your 
correspondent alludes, viz. the neighbourhood of Bath, and 
within a few miles of Mells, I never remember to have seen or 
