Fuly 7, 1870] 
NATURE 
but I see no @ friovi reason for expecting to find one such law 
rather than another ; we must try which assumed law will most 
nearly coincide with fact, and the hypothesis of a harmonic 
mean does so coincide pretty nearly. The following table (see 
my previous letter) gives the ratios of the wave-frequencies of 
red, orange, and yellow as observed, of their complementaries 
as observed, and of the same as calculated on the hypothesis of 
the harmonic mean :— 
Observed. Calculated. 
Red... 364¢ Bluish Green 48°30 48°53 
Orange . 39°80 Ble sae. 52780 53°07 
Yellow . 41°40 Indigo... . 54°70 55°20 
The discrepancies between the observed and the calculated 
outlines are much less on this hypothesis than in that of the 
geometric mean ; but they are on the same side, and, as I ex- 
plained in my former letter, I think it likely they may be due to 
the solar spectrum not being of a truly white colour, owing to 
the absorption lines toward the violet end. They are on the side 
which this way of accounting for the fact requires. It would be 
desirable to make a set of comparative experiments with solar 
light and the electric light, asI suggested before, in order to 
clear up this question. 
Old Forge, Dunmurry, 
: JosEPH JouN MurpHy 
Co. Antrim, June 13 
On the reported Current in the Suez Canal 
Ir is stated on excellent authority thata constant current runs 
threugh the central portion of the Suez Canal, from the side of 
the Mediterranean to that of the Red Sea, and a good deal of 
surprise has been excited by this apparently anomalous phe- 
nomenon. A little consideration will, however, suffice to 
establish a theory, that constant currents are almost necessary 
conditions of inter-oceanic canals, and that their absence, not 
their presence, would be contrary to just expectation. My reason 
is based on the improbability that a long canal, A B, could be 
constructed across strata that are almost necessarily inclined in 
one direction more than another, which should not resist the 
flow of tidal water from, say, A towards B, more than from B 
towards A. Wherever this differential aspect is established, a 
quasi-valvular action is called into existence, and a current along 
the middle of the canal, in a constant direction, is the necessary 
consequence, 
Let A B be the canal, and a 6 the extreme limits of tidal 
influence. After each successive rise and fall of the tide on 
either side, more water will have passed from A towards @, than 
A a b B 
will have returned from the side of a to A, and more water will 
be able to travel from the side of 4 to B, than can get up the 
canal from B towards J. Consequently there will be a constant 
current in the ultra-tidal portion, @ 4, of the canal, from the side 
of A to that of B. 
I have made some inquiries, but am unable to learn what 
notchings, indentations, or sweeps of the sides of a canal, would 
exercise the greatest differential effect, at low velocities, of the 
kind of which I am speaking. However, I hear it is a fact 
well known to sailors, that a spar cannot be towed behind a 
boat, unless with the greatest d fficulty, if its small end be fore- 
most, whereas, it is moved easily enough if its thick end be in 
front. I argue from this that ifa number of spars were moored 
against the sides of the canal, with their larze ends towards A, 
much less strain would be exerted on the ropes by which 
they were secured when the current ran from A to B, than when 
itran from B to A, and consequently that the current itself would 
be much less resisted in the former than in the latter case. A 
succession of very long notches in the sides of the canal would 
produce identically the same effect, and might call into existence 
a considerable aggregate of differential resistances. I constructed 
a model for the purpose of experiment, but found it much too 
small to give satisfactory results ; nevertheless, I will describe it, 
in hopes it may save trouble to others in designing a suitable 
arrangement, for the same purpose, on a larger scale. A 
notched trough was cut, running up and down in long zig- 
zags, and its two ends were brought together into the same 
reservoir. By alternately allowing water to run into the 
reservoir, and then drawing it off, the effect of the rise and 
fall of the tide was simulated. I scattered lycopodium on 
189 
the water, in the middle part of the channel, to show the direc- 
tion of the current. 
I venture to suggest to those engineers who are connected 
with inter-oceanic canals, the importance of making experiments 
on this problem, because it may prove to be quite within their 
means to produce and to regulate a current within such canals, 
in the direction and of a velocity most convenient to keep its 
bed clean and serviceable. FRANCIS GALTON 
Birds’ Nests 
Birps, though almost always adapting themselves to cir- 
cumstances in the use of materials, are frequently, even in the 
country, very eccentric in their choice of a place for their nests. 
I have scen a blackcap’s nest built of the ordinary materials, in 
an open flower-pot standing on the top of a garden wall. 
Apparently there was no possible reason for this, there being 
plenty of hedges and banks hard by. But in the neighbourhood 
of London birds may be allowed an excuse for their eccentricities. 
In a quiet street in one of the southern suburbs there is now a 
pair of tom-tits who have taken possession of a cast-iron lamp 
pillar, wherein they have built their nest and reared their young 
for two or three years past. It is curious to think what business 
they could have had there, to have found out that it was a suit- 
able residence. The nest is placed in the bulb or swelling out of 
the column, just below the lamp, and the birds creep through 
the space between the gas-pipe and the iron rim at the top of the 
ated This space is not three-quarters of an inch in width. 
The nest is on one side of the pipe, and cannot be more than 
two inches across. The lamp is lighted every evening, and on 
one occasion the pillar was actually taken down for some repairs 
with the nest inside, containing seven or eight eggs, which were, 
I believe, destroyed ; but the birds, concluding I suppose that 
this was not done with malice prepense, but that it was only a 
necessary domestic difficulty, wisely returned to their home, and 
continued to occupy the lamp pillar for the remainder of the 
season, rearing another brood that same year. The accompany- 
ing sketch shows the position of the nest. Under the eaves of 
the adjacent house, two pairs of house martins have built this 
year. They came flitting about on the Ist June, and the weather 
being very dry, and no mud to be got at, the ‘‘ gudeman” of the 
house kept a little spot in the road well watered, from whence 
the birds obtained all their necessary mud. The sparrows would 
plump down after the martins, thinking there was food there, 
and stand watching the martins at this little wet spot, and won- 
dering apparently why they kept on flying down here, where 
there was no grub to be got. “Vhey tried hard to obtain posses- 
sion of the martin’s nests when half built, but were constantly 
driven away by the gentleman of the house, and now the nests 
are finished, and the entrance too small for the sparrows to get in ; 
so that they dwell in comparative security. Two of these martins 
seem to be the sole occupants of their nest, but the other nest 
appears to be visited, at least, if not owned by, more than one 
pair of birds, three or four birds being often seen there at one time. 
I have often noticed this in the country, but never saw any 
remarks about it recorded by any one. 
CW. NW. 
