1895 - 96 .] 
Chairman's Opening Address. 
7 
■used by him to measure the vapour in the atmosphere by weighing 
the water extracted from it when dried. Experiments were carried 
on at Ben Nevis Observatory, at Fort-William, and at Montpellier 
by Mr Herbertson and several assistants. The results obtained at 
these places agree so well that they have enabled the Author to 
construct tables showing the relations of vapour-pressure, atmos- 
pheric pressure, and temperature throughout a considerable range. 
He also discusses the reduction of the readings of the Dry and Wet 
Bulb Hygrometer, as given by different authorities, shows wherein 
these differ, and how far the present set of experiments throws 
light on this important question in practical meteorology. 
Mr J. C. Beattie has given us a paper on the Belation between 
the Variation of Besistance in Bismuth in a Steady Magnetic Field, 
and the Rotatory or Transverse Effect. One important feature in 
this paper is the novel mode of experiment, the magnetisation being 
determined by the resistance change, so that the study of the Hall 
Effect is materially simplified. The anomalous behaviour of 
Bismuth is carefully studied under various conditions, and after 
being subjected to varied mechanical treatment. 
This Society has, for many years, been particularly strong in 
mathematical and physical experimental science, and I think it 
will be allowed by those who have followed the work of the past 
two Sessions that our reputation in this respect has been fully 
maintained. Professor Chrystal has communicated a fundamental 
theorem regarding the Equivalence of Systems of Ordinary Linear 
Differential Equations, and its application to the Determination of 
the Order and the Systematic Solution of a Determinate System of 
Equations, and Professor Crum Brown has shown us, by means 
of a series of elaborate and ingenious models, how a Parallelopiped 
can be divided into Tetrahedra. 
In his paper on the Kinetic Theory of Gases, Professor Tait 
pointed out, some years ago, that the celebrated equation of Van 
der Waals had been obtained by an unsound process, and thus 
could not be correct. He found it inconsistent with the results of 
his “ Challenger ” wmrk on the compression of water under great 
pressure. In a recent paper in our Proceedings he returns to this 
question, basing his tests upon the lately-published results of 
Amagat’s splendid work. The comparison is curious. From Van 
