ogo 
be determined whether either the safety of the machine 
or the comfort of the passenger requires a modification 
of the stability. 
Messrs. Bairstow and Nayler’ have analysed the 
motion for a complete minute of an aeroplane moving 
over the ground with steady speed of 60 ft. per sec. 
in a wind as registered on an open-scale record of velo- 
city. changes, obtained at Kew Observatory. The 
velocity of the wind ranged from 11 to 33 ft. per sec., 
the average being 20 ft. per sec. Curves are given in 
the paper showing the changes in the wind velocity 
during the minute, and the variation in the velocity 
of the air relative to the machine during the same 
minute. The similarity of the two curves is marked. 
Curves are also given which show that if the speed of 
the aeroplane over the ground in still air is taken as 
80 ft. per sec., its speed relative to the gusty air (as 
shown in the anemogram referred to) varies from 
70 to 94 ft. per sec. The aeroplane has not time to 
respond to the rapid changes in the wind. While the 
changes in the actual horizontal velocity of the aero- 
plane are considerable, they occur much more slowly 
than in the wind-velocity curve; the minor alterations 
are wiped out; a rise in the wind velocity causes a 
fall in the velocity of the machine, provided the 
changes are sufficiently prolonged, but a very rapid 
rise and fall of the wind velocity is scarcely noticeable. 
It is assumed that the controls have not been touched 
while this motion is in progress. Curves are, how- 
ever, also given, showing the effect of altering the 
elevator during the gust, and it appears that the 
elevator can without difficulty be so manipulated as 
practically to cancel the effect of the gust. The curves 
deal only with the longitudinal motion of the machine. 
Messrs. Bairstow and Nayler are now engaged in the 
similar problem for the lateral motion of the machine, 
and when this is completed, propose to attack in the 
same way the motion of a biplane of standard form. 
The practical outcome of work of this kind is shown 
in the Army aeroplane R.E.1. The importance of this 
machine arises from the fact that it was designed 
to have inherent stability as the result of calculations 
based on scientific experiments, such as have been 
described in this lecture. 
The Advisory Committee for Aeronautics has given 
much attention lately to the consideration of the 
stresses to which a machine may be subject in flight. 
The normal stress coming on any part of the machine 
is usually taken as that which it has to bear in steady 
horizontal flight, produced, that is, by a loading equal 
to the weight of the machine; if the breaking stress 
is N times this, N, according to present usage, is 
called the factor of safety. A machine, however, in 
its ordinary use may frequently have to carry a load 
much in excess of what it bears in steady horizontal 
flight. It would be more consistent with engineering 
practice to estimate what is the maximum stress the 
machine in its daily use may have to bear, and then 
take as the factor of safety the ratio of the breaking 
stress to this maximum stress. The factor of safety 
would thus take account of imperfections of workman- 
ship or of material, not of varying load. If the 
maximum stress to be allowed for is taken to be equal 
to a loading N, times the weight of the machine (the 
normal loading in horizontal flight), and the breaking 
stress is n times this, then the ratio of the breaking 
stress to that occurring during steady horizontal flight 
is mN,. This is called N, so that N=nN,, and N, 
not n, is the factor of safety as ordinarily but mis- 
takenly used in aeronautics. ; 
The value of N has been determined by calculation 
and, in some cases, by direct experiment, for a number 
of machines, and: appears to range from 3 to 7 or 
more. It is shown that a sudden gust may cause 
stresses on a machine four times as great as those 
NO. 2328, VOL. 93] 
NATURE 
| 
[JUNE II, 1914 
occurring in steady horizontal flight at maximum 
speed. Another cause of serious sudden increase in 
loading is rapid flattening out after a dive, and cal- 
culation shows that stresses from eight to ten times 
those due to normal loading may be experienced due 
to this. From a consideration of these figures it is 
clear that it is essential to make an effort to strengthen’ 
machines so that N,, the load factor, is at least six. 
Giving n the value of two (although an engineer 
would certainly think it too low for his work) the 
value of N would be twelve. There are great difficul- 
ties in attempting to reach so high a value at present, 
but it is not thought that the degree of safety specified 
is beyond reach. 
THE METRIC SYSTEM.* 
INCE its introduction into the United Kingdom 
the metric system or question has had its ups and 
downs. Surely it is very curious that, although in 
1862 a Parliamentary Commission recommended its 
introduction—a recommendation since repeated two or 
three times—and that a Bill was actually passed by 
the House of Lords, the metric system has not been 
adopted in this country. Why do people go on agitat- 
ing? Well, the reason is the necessity for such a 
system. The facilities for intercommunication between 
various countries have a great deal to do with the 
continual agitation to introduce an_ international 
system of weights and measures. You may say the 
first person who put this down in black and white was 
James Watt. Writing to a friend in 1783 he said it 
was very awkward that the scientific results of workers 
in various countries could not be compared readily 
because of the measurements and weights being so 
different, and he proposed that they should agitate 
for the adoption of an international unit of weights 
and measures for scientific purposes. He wrote to 
French savants on the subject, and the result of the 
agitation was that in 1790 Prince Talleyrand brought 
in a Bill before the Legislative Assembly of France 
proposing that a Commission should be nominated to 
deliberate on this subject. It was a provision of that 
measure that the Royal Society of London and the 
French Academy should nominate the members of the 
Commission because it was agreed that the Com- 
mission ought to be an international affair and not 
merely a national one. The Royal Society would not 
agree to it because, as you know, England and France 
were at war at that time. Eventually, however, some 
other countries joined and constituted a Commission. 
Another feature of the metric system was also sug- 
gested by Watt. He suggested that the unit of 
length should be cubed, a vessel constructed, filled 
with water at its greatest density, and that that should 
be the unit of weight. This cube should be the unit of 
capacity. In carrying out this idea insuperable diffi- 
culties have arisen of an absolutely mechanical nature, 
and so a kilogram is not any more a decimetre cubed 
and filled with water, but it is a piece of platinum 
kept in Paris at a certain temperature and at a cer- 
tain barometric pressure. But the difference is very 
slight and does not affect the value of this co-relation 
between length, capacity, and weight. That is just 
the same as the standard of British measure—in fact, 
the real standards of English weights and measures 
were burned in 1835 in the Houses of Parliament 
and had to be reproduced afterwards as best they 
could. Secondary standards have now been made 
and have been distributed over the country, so that 
there is no danger of the standards being lost again. 
After giving you this short history of the beginning 
1 From a report published by the Decimal Association of an address to 
the members of the Bradford Textile Society and of other Trade Organisa- 
tions at Bradford, on November 17, 1913, by Mr. Alexander Siemens. 
