we wT 
SECTIONAL TRANSACTIONS.—A. 365 
observations made among trees. The temperature gradient was measured by a 
pair of thermo-junctions placed at different heights. This is compared with 
the gustiness as shown by a Dines pressure-tube anemometer. 
23. Dr. H. Jerrreys.—Tidal Friction. 
24. Discussion of observations, &c., made by Meteorological party 
during journey from England to Canada on R.M.S. Caronia. 
(a) Mr. M. A. Grsterr.—Remarks on the Daily Weather Charts 
constructed from data received by Wireless. 
(b) Mr. L. F. Ricnarpson.—Measurements of Up-gradient of 
Temperature in the Air at Levels below the Mast-head of the 
Ship, using an Electrical Resistance Thermometer. 
(c) Mr, F. J. W. Wuiwers.—The Measurement of True Air 
Temperature and Humidity at Sea. 
A comparison of readings of screened and unscreened thermometers with 
those of an Assmann aspiration psychrometer in various positions on the ship. 
(d) Dr. J. 8. OwEns.—Haze Observations. 
A white haze observed on the St. Lawrence in the neighbourhood of Anticosti 
Island shortly before sunset was found to consist of hygroscopic salt particles, 
although readings of an Assmann psychrometer showed the air to be relatively 
dry. [An apparently similar haze observed later over Lake Ontario was found 
to consist of solid particles, almost certainly soot. ] 
Monday, August 11. 
25. Presidential Address by Prof. Sir Witi1aMm Braae, F.R.S., on 
The Analysis of Crystal Structure by X-rays. (Page 34.) 
26. Joint Discussion with Section B on Crystal Structure, including 
the following papers :— 
(a) Prof. W. L. Brace, F.R.S.—The Relation between Crystal Structure 
and Refractive Index. 
When an electromagnetic wave passes through a medium the electric fielc 
polarises the atoms, their positive and negative components being displacec 
from the normal configuration. This creates a field around each atom equiva- 
lent to that of an electric doublet whose moment is proportional to the electric 
vector at each instant. The velocity of the wave in the crystal depends on the 
ratio between the total polarisation per unit volume and the field. 
Each atom is affected by the general field and by its polarised neighbours. 
In a crystal which has been analysed by X-rays the effect of neighbouring atoms 
on each other can be calculated. The double refraction of Calcite and Aragonite, 
for instance, can be satisfactorily explained quantitatively. This holds for 
other crystals, and similar calculations are of interest when applied to gases. 
It appears probable that a better measure of atomic refractivity will be 
arrived at when due allowance is made for atomic arrangement. 
(b) Prof. C. H. Duscu, F.R.S.—The Crystal Surface. 
The atoms in a crystal being arranged on a space lattice, all atoms in the 
interior of the crystal must be held in position by forces symmetrically disposed. 
At the surface of the crystal this symmetry disappears, so that the surface layer 
of a crystal must possess properties different from those of the mass. A part of 
