_ . 
Fan. 4, 1883] 
NATURE 
223 
struts, even if arranged as he first conceived them to be, could 
not have failed by flexure during the wildest hurricane. 
4. Sir George finds fault with the connection of the brackets, 
and ‘‘can hardly imagine that trains could be run through at 
speed.” I should have been pleased to have explained the con- 
nection to Sir George, but he has not sought to know anything 
about the details of the bridge, and, I am sure, would be much 
puzzled to give your readers even the vaguest possible description 
of the connection, which he nevertheless stigmatises as ‘‘ not 
very perfect.” 
In conclusion, as Sir George has not done so himself, I would 
warn any young student who may have read the investigation 
contained in the appendix to the fir-t letter, that the methods 
therein proposed would lead to an over-estimate of the strength 
of struts of ordinary proportions by from 200 to 300 per cent. 
This warning is the more necessary, as the general tenour of Sir 
George Airy’s letter might make a student imagine that he erred, 
if anything, in the direction of excess of caution, whereas the 
application of the principles laid down by him would, in the 
case of the Forth bridge, result in the compression memfers 
being made only one-third of the strength considered expedient 
by Mr. Fowler and myself. B. BAKER 
2, Queen Square Place, Westminster, S.W. 
P.S.—It may interest some of your readers to know that the 
maximum force recorded during recent storms by our wind 
pressure plates at the Forth has been 20 lbs. per square foot, 
upon the small and light plate having ian area of 2 square feet, 
and 12} lbs. upon the large and heavy one, with an area of 
300 square feet. The same ratio holds good down to pressures 
of 2 lbs. per square foot, and it appears pretty certain that the 
higher blasts are of such momentary duration and of such 
unequal distribution, that even a small sized railway bridge 
could never experience ordinary anemometer pressures. Other 
reasons for a reduced pressure on a large surface have been ad- 
vanced by Dr. Siemensin a recent number of the Comptes Rendus. 
Nevertheless, in this instance of the Forth bridge we have assumed 
that a 56 lbs. hurricane will act simultaneously over the whole 
width of the Forth, with a resultant lateral pressure of no less 
than 8000 tons upon the main spans. We have further assumed 
that the said hurricane might blow down one side of the Forth, 
whilst a dead calm prevailed on the other side, and have even 
provided for the twisting action upon the piers and superstruc- 
ture due toa 56 lbs. hurricane blowing #f the Forth on one 
side, and dow7 it on the other. To ascertain what lateral pressure 
a 56 lbs. hurricane would cause, we tested, both in air and in 
water currents, a large model of the bridge, with cross-bracing 
complete, and ascertained its equivalent in square feet of flat 
surface. Under any of the conditions of wind pressure enume- 
rated above, combined with any distribution of the rolling load, 
the resultant stresses upon superstructure, holding down bolts 
and piers will be far within the safe working limits as determined 
by our experiments upon the respective materials.—B, B. 
Altitude and Weather 
In NATuRE, vol. xxvii. p. 176, you notice the remarkable 
warm and dry weather September 21 last on Ben Nevis, during 
an anticyclone, and, as at the foot the air was relatively cold 
and humid, you see in the heat and dryness on the mountain an 
effect of descending air currents. In this you are quite right, but 
I do not think you are right in estimating that this air was 
saturated at a certain height above Ben Nevis. The fact is this: 
the increase of temperature from a certain height above sea-level 
to the latter being ¢e facto much less than the dynamical increase 
of a stratum of air, due to compression sinking down, a down- 
ward current of air will be generally warm, and relatively dry. 
It does not matter if it sinks along the slopes of mountains (as 
the foehn), or vertically, as modern meteorology considers it to 
be the case in anticyclones. There is only one great difference : 
the air currents down a slope may be, and often are, very 
violent, and only when they are so, their relative heat and dry- 
ness are felt, while the downward currents in an anticyclone are 
so gentle that they are seldom felt or directly registered, and 
that mostly the thermometer and hygrometer are our only means 
of detecting them. On account of their slow motions, the effect 
of these downward currents during anticyclones is little felt in 
valleys and plains, as (1) they are even more retarded near great 
nd surfaces ; (2) in the colder time of the year, especially when 
the ground is covered with snow, the radiation from the soil 
lowers the temperature of the lower strata. Thus during anti- 
cyclones in winter a very low temperature is generally experienced 
in plains and valleys, due to radiation, and a very high tempera- 
ture and low humidity on isolated mountains, due to descending 
currents of air. 
These conditions are best realised during protracted and 
considerable anticyclones, and it was Prof. Hann’s merit to have 
explained this fact.' The exceedingly protracted anticyclone of 
December, 1879, in Central Europe, was especially favourable 
to the proof of the existence of descending currents, as the cold 
was great in the valleys, even in the high ones, like the Engadine 
and the Davos, but the air was warm and very dry on isolated 
mouvtains, An example from the best mountain observatory 
of that time, the Puy de Dome, and the foot of it, will suffice, 
uine days, December 20-28, 1879, at 6 a.m. 
Relative Amount of 
Feet. Temp. F. humidity Cloud 
Puy de Dome, 4813 38°38 eto bce gy 35) 
Clermont (base) 1273 Soe. | On o7 
There is all reason to think that in these days there was no 
sa!urated stratum of air even considerably above, say the Puy de 
Déme. 
I must remark that Prof. Hann, in his last work, “ Der Fohn 
in Bludenz,” dces not sustain his former opinion that great 
precipitations on the windward side of mountains is necessary to 
the appearance of a foehn on the leeward side. His opinion 
now is, that a considerable barometric gradient and the drawing 
in of air from considerable heights are alone necessary, for even 
if the air on the mountains is not abnormally warm, it will come 
down warm and relatively dry. A. WOEIKOFF 
Ofizerskaja, St. Petersburg, December 15-27, 1882 
The Fertilisation of the Speedwell 
I FEAR that Dr. H. Miiller’s passage in Schenk’s ‘‘ Hand- 
buch ” would occupy too much space to be given here in full ; 
but I can condense what he says into a few lines. Dr. Miiller 
takes the Veronica chamedrys as representing a type of flowers 
in which the anthers have to be brought into a position to strike 
the body of the insect by the action of the insect itself. He 
finds the same arrangement in the V, ustic@folia. These flowers 
are visited by insects of various kinds, but their structure is, he 
thinks, explained only by what takes place when they are visited 
by Syrphide. When one of these insects visits such a flower, 
it hovers for some seconds before it, then settles upon the lower 
lobe of the corolla, without noticing the style which is coloured 
like the corolla, and which is now under the insect’s body. It 
then crawls higher to reach the nectary, and in doing so bends 
down the stamens—which are also coloured like the corolla— 
until the anthers strike against the under part of the insect’s 
body. The pollen thus obtained is carried to another flower, 
and brought into contact with the stigma when the insect first 
alights ; and fresh pollen is again obtained by the attempts to 
reach the nectary. Dr. Miiller either knows from observation 
or assumes that in the V, chamedrys anthers and stigma are 
mature at the same time. He attaches importance to the fact 
that both stamens and style are coloured like the corolla, and 
therefore appear to escape the observation of the insect ; and the 
thinness of the base of the stamen is also noticed by him as one 
feature in the adaptation of the flower to the visits of syrphide. 
He does not refer to the looseness of the corolla, Mr. Stapley’s 
suggestion that this may play some part in the work of cross- 
fertilisation is an ingenious one, and calls for further research. 
As to the V.heder@folia, Dr. Miiller mentions it as one of the 
plants that have a tendency to keep their flowers half-shut in 
cold and rainy weather, and thus to become cleistogamous. 
I am sorry that I misunderstood Mr. Stapley’s first letter upon 
any point ; but he has misunderstood mine also, if he thinks I 
was not aware he wished to call attention ‘‘to the adap/ation of 
the flower for cross-fertilisation.” I wrote as briefly as I could, 
and naturally assumed that he would understand I was not 
thinking merely of the fact that Diptera drew down the 
stamens. ARTHUR RANSOM 
Bedford, December 23, 1882 
THE SACRED TREE OF KUM-BUM 
7 ee dissipation of illusions is always a little painful, 
even after repeated experience of the process. I 
must confess, then, to some feeling of injury at learning 
from Mr. Keane’s interesting review in NATURE, vol. 
1 Zeitschr. fiir Meteorologie, p. 129, 1876. 
