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TRANSACTIONS OF SECTION A. 419 
mercury light between Nicols set to extinction, and then to examine it by 
means of a telescope focussed on the interior of the crystal; (2) to read its 
optical rotatory power with mercury-green light, using both a positive and a 
negative half-shadow angle. Rods cut from a flawless crystal-plate of levo- 
quartz weighing about two kilos. gave an optical rotation of 25°-5371 per mm. at 
20° C. for A =5461 on a total length of 226 mm.; rods cut from a plate of dextro- 
quartz, containing local faults, gave the figure 25°°5361 per mm. on a total length 
of 181 mm. Observations taken with light of twenty-four different wave- 
lengths showed errors of reading amounting on the average to only six parts per 
million; the errors from all sources: were probably not greater than 0°-001 to 
0°002 per mm., as the average deviation from values calculated from a formula 
was only 0°-0015 per mm. 
5. Calibration of a Wave-length Spectroscope in the Infra-red Region 
of the Spectrum. By T. Martin Lowry, D.Sc., F.C.S. 
A wave-length spectroscope, provided with a slit, thermopile and galvano- 
meter instead of an eyepiece, was calibrated (a) by plotting the maxima and 
minima of the light transmitted from a Nernst lamp through a lightly silvered 
etalon; (b) by locating the principal infra-red lines in the spectrum of the mer- 
cury arc. The line at A 10,140 coincided with the fourteenth maximum of the 
etalon, and was used as the principal fixed-point in the calibration. The 
calibration was probably correct within 54, to ;{, of a drum-revolution from 
A 8,000 to A 17,000, corresponding to a possible error of 20 to 50 Angstrém units. 
From A 17,000 to A 20,000 the calibration was less exact, and the errors may 
have amounted to 100 units. 
6. Registration and Analysis of Sound Vibrations. 
By Professor D. C. Miter. 
DEPARTMENT OF METEOROLOGY. 
The following Papers and Reports were read :— 
1. Recent Investigations of the Temperature of Fresh-water Lakes. 
By E. M. Weppersery, F.R.S.E. 
_ At the Dublin meeting of the Association in 1908 the author communicated 
a paper describing certain oscillations of temperature in Lochs Ness and Garry, 
to which the name of Temperature Seiche was given. He has since been con- 
cerned in observations in the Madiisee, Pomerania, and Loch Earn, Perthshire, 
undertaken in order to further establish the oscillatory nafure of the changes 
observed. The Madiisee observations were conducted jointly with Professor 
Halbfass, Jena, who did not admit that the temperature seiche was likely to 
be found save in deep lakes situated in deep glens, such as Loch Ness and Loch 
Garry. But a temperature seiche was found in the Madiisee, which is situated 
in a flat country. The shores everywhere are of a shelving nature, and there- 
pe the conditions are quite different from what is usually found in Scottish 
ochs. 
The observations in Loch Earn were on a more ambitious scale, for two-hourly 
(sometimes hourly) observations were made at five points along the loch for 
about four weeks. A record was kept of meteorological data, and in particular 
a pressure-tube anemograph was used for recording the strength of the wind. 
In this way it was frequently possible to explain variations in temperature by 
reference to meteorological conditions. During part of the period of observa- 
tion a good example of a temperature seiche was observed, and it was possible 
to distinguish uninodal and binodal periods. The observations at each end of 
the loch showed clearly the opposition in phase of the uninodal ‘seiche, and 
EE 2 
