410 SECTIONAL TRANSACTIONS.—A. 
heard of equal strength with the open end of the box pointing towards, or 
directly away, from Paris; but ceased when the box was turned so that the 
open end faced at right angles to Paris, the frame still pointing to Paris; or 
when the open end was completely closed with a copper or tinfoil cover. In 
the latter case the sigrals were still audible unless the cover actually touched 
the box on all sides. For other positions of the box with the end open, signals 
could only be heard when the relative positions of the box and frame were such 
that a prolongation of the plane of the frame towards or away from Paris, no 
matter which, came out of the open end clear of the copper sides of the box. 
8. Sir F. W. Dyson, F.R.S.—Results with the 72-in. Reflector in 
British Columbia. 
9, Prof. H. Briaas.—Prehensility: a Factor of Gaseous Adsorption. 
Prehensility is defined as the slope at the origin of an adsorbent isotherm. 
A method of measuring prehensility is described, and results given for 
various adsorbents at liquid air temperature. 
It is shown that from the prehensility the evacuating power of a substance 
may be calculated. In evacuating any given volume, the weight of charcoal 
required to yield a required reduction of pressure may be computed. The 
degree of vacuum obtained in a Dewar liquid air container is discussed. 
The plum-stone charcoal used by the author had a higher evacuating power 
than coconut charcoal. 
Reference is made to a colloidal silica of appreciable evacuating power, 
though, at —190° C., over four times as much of it is required (by volume) as 
of plum-stone charcoal to attain the same result, and it acts more slowly. 
The very high degree of vacuum procurable by using a succession of charcoal 
bulbs is discussed, and it is shown that with a given weight of charcoal the 
reduction of pressure obtainable by division of the mass among a number of 
bulbs does not indefinitely increase with that number, but eventually reaches 
a maximum. 
(b) DEPARTMENT oF CosmicAL Puysics. 
10. Dr. H. Jerrreys.—The Cause of Cyclones. 
11, Capt. C. K. M. Doucuas.—Some Remarks on Bjerkne’s Theory of 
Cyclones and Anticyclones. 
The following demonstrations were given during the afternoon :— 
12. Mr. J. J. Dowiinc.—Demonstration of the Recording Ultra- 
micrometer and some of its Applications. 
(1) Theory.—Variation of the capacity in an oscillating valve circuit causes 
variation of the plate circuit current. (2) A potential balancing device enables 
a sensitive galvanometer to be employed to record these changes. (3) Extremely 
minute changes in capacity due to relative displacements of plates (10-9 cm. or 
less) are readily detectable. (4) The sensitivity is independent of the condenser 
plate distance over large ranges (up to 1 mm.). (5) The device is quite stable 
and consistent in operation. (6) The following applications were exhibited :— 
(a) Micrometer arrangement for demonstration. 
(6) As applied to a seismograph; specimen records. 
(c) Micro-pressure manometer. 
(d) Rapid action sensitive balance. 
(e) Application to recording of plant growth. 
(7) Other applications are under investigation, including the measurement of — 
variation of gravity—it is quite easy to detect the diminution in gravity due to 
a height of one metre. 
13. Mr. W. L. Batus.—Demonstration of Simple Harmonic Analyser 
and Periodoscope. = 
The apparatus consists of a set of equidistant strings under equal tension 
radiating from a flexible yoke, the movements of which are observed by an 
optical lever. The angle of divergence of the strings is small, so that they may 
be considered as being parallel. 
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