424 
incidence greater than about 78°, and this is unfor- 
tunately a considerably larger angle than the probable 
polarising angle. Experiments with incidence in the 
neighbourhood of 45° should prove peculiarly decisive, 
for whereas ordinary light cannot as a rule be com- 
pletely polarised by reflection, the reflection of X-rays, 
which occurs at planes of atoms, is independent of 
any contamination of the exposed crystal surface, and 
polarisation, once established, should prove complete 
for radiation reflected at the polarising angle. The 
selectively reflected X-rays seem to show the same 
effects as does the generally reflected beam. Selec- 
tively reflected radiation is always detectable after 
the second reflection, but this seems due to the selec- 
tively reflected radiation produced at the second 
reflector by the unpolarised portion of the beam gener- 
ally reflected at the first reflector. 
The application of a theory of polarisation to 
explain the above results is interestingly supported 
by the fact that in the case of two reflections by 
parallel reflectors, the proportion of X-rays reflected 
at the second reflector is invariably greater than the 
proportion of rays reflected at the first; that is, the 
ratio of reflected radiation to incident radiation at the 
second reflector is always greater than the same ratio 
at the first reflector. This might be expected if vibra- 
tions perpendicular to the plane of incidence are to be 
reflected to a greater extent than those in the plane 
of incidence. The proportion of such vibrations is 
larger in the beam incident on the second reflector 
than in the original beam, and a greater proportion 
of radiation would be reflected at the second reflector 
than could be at the first. For the case of parallel 
reflectors and incidence of a primary beam on the 
first at the polarising angle, the reflection at the 
second should be complete. E, Jacor. 
South African College, Cape Town, 
November 14. i 
Residual lonisation in Gases. 
From observations made by Simpson and Wright, 
the writer, and others, it is now known that the 
ionisation in air confined in airtight clean zinc vessels 
is about 8 or g ions per c.c. per second when the 
observations are made on land where the soil contains 
only such minute traces of radio-active substances as 
are found in ordinary clays or loams. 
On the other hand, when the observations are 
made on the ocean or on the surface of large bodies 
of water, such as Lake Ontario, the ionisation in the 
same air confined in the manner indicated above drops 
to about 4 ions per c.c. per second. This reduction 
in the number of ions per c.c. per second has been 
shown to be due to the absorption of the earth’s 
penetrating radiation by the water of the ocean and 
by that of the lakes. 
On a recent voyage from England to Canada, I 
thought it would be interesting to see what the drop 
in the ionisation would be when the air in a zine 
vessel was replaced by hydrogen. The observations 
were made on the ss. Megantic, a vessel of about 
14,000 tons burden. On this boat the ionisation in 
air confined in a Wulf electrometer made of zinc was 
found to be 4-65 ions per c.c. per second, while in 
hydrogen it was 1-8 ions per c.c. per second. On 
reaching Toronto the experiment was repeated in a 
building which was free from any radio-active im- 
purity, and in this case the ionisation in air was found 
to be 88 ions per c.c. per second, while in hydrogen 
it was 2-0 ions per c.c. per second. 
The ionisation of the air on land was _ there- 
fore 4-15 ions per c.c. per second more than it was 
upon the steamship, while the ionisation in hydrogen 
on the land was only o-2 ion per c.c. per second 
NO. 2302, VOL. 92] 
NATURE 
[DECEMBER II, 1913 ~ 
more than on the sea. From this it follows that the 
ionisation produced in air by the penetrating radia- 
tion at the surface of the earth at Toronto was about. ay 
twenty times as much as that prOduced by the same — 
radiation in hydrogen. a, 
Since the residual ionisation in hydrogen on_ the 
ocean was nearly 40 per cent. of that in air, it 
evident that the residual ionisation in these two gasi 
could not have been due to a radiation of the type 1 
origin in a disruption of the molecules occurring either 
spontaneously or through the agency of collisions. 
J. C. McLennan. 
The Physical Laboratory, University of Toronto, 
November 13. 
The Nile Flood of 1913. 
For some years past the Meteorological Office of 
the Egyptian Survey Department, under the direction 
of Mr. J. I. Craig, has carried out researches on the 
question of the possibility of forecasting the Nile flood, 
and he has put forward the theory that the rain which 
falls in Abyssinia comes from the South Atlantic (see 
“England, Abyssinia, the South Atlantic: a Meteoro- 
logical Triangle,’ Quarterly Journal Royal Meteoro- — 
logical Society, October). a 
There is much evidence to support this, and cor- 
relations have been established between the flood, 
and pressure and wind velocity at St. Helena, and 
pressure in South America. So far the best pre- 
diction which can be based on these correlations is for 
the mean height of the Nile at Halfa, between July 16 — 
and August 15, that is, in normal years for the middle 
of the rising stage. The probable error of a pre- 
diction based on this is +033 metre, whereas a 
prediction which assumes that the river will be normal 
in any given year would have a probable error of 
+055 metre. This result is sufficiently encouraging 
to make further work promising, and the writer is 
pursuing the investigation. 
The flood of this year has been the lowest of which 
there is authentic and complete record. Records of — 
the maximum and minimum of the flood as recorded 
on the Roda Nilometer (Cairo) go back to very early 
times, but naturally the early ones are less trustworthy 
than those of more recent date. The following figures, 
taken from ‘‘ Egyptian Irrigation,’ by Sir William 
Willcocks and Mr. J. I. Craig, give the lowest 
recorded floods in recent times :— 
Lowest maximum No. of years 
Period recorded on Roda of period 
Ni lometer recorded 
Metres 
1701-1725 17-35 18 
1726-1750 18:58 24 
1751-1775 18:08 25 
1776-1800 15:49 (1 
1801-1825 13°14 (2 ‘oes 
1826-1850 18-15 25 
1851-1875 18-30 25 
1876-1900 a8 17-65 462 25 
IQOI-I913 ; 17-17 (1913) -.- 13 
(2) Is almost certainly an error of to pics. (the 
divisions on the gauge), and it seems very probable 
that (1) is also an error, as at the present day in the 
low stage the river is artificially kept at a level of 
about 15 metres by the Delta Barrage and the Aswan 
Dam, and the average level in the low stage before 
the Barrage became effective was about 13 metres. 
During the last twenty-four years calculations of 
the discharge at Halfa heve been made, and as the 
