of Edinburgh, Session 1874-75. 623 
above its mean more than one-half of the amount to which it falls 
below it. When below the mean, equatorial winds prevail, and 
the greater range of pressure which then takes place I attribute to 
the cause above stated. 
Note . — In the “Philosophical Magazine” for September 1874, 
Mr Tylor comes to the conclusion that “ the barometer cannot 
give a true indication of weight when there is motion in the 
atmosphere.” 
2. On Electric Images. Professor Tait. 
3. Laboratory Notes. By Professor Tait. 
a. On the Origin of Atmospheric Electricity. 
This was a preliminary notice of the results of a series of ex- 
periments devised to test the part played by water-vapour in the 
production of atmospheric electricity. While water is in the form 
of vapour it must be electrified by contact with the gases of the 
atmosphere — as they are by contact with one another. Precipita- 
tion of vapour in a receiver, whether produced by cold or by 
exhaustion, was found to be steadily accompanied wrth a dis- 
engagement of electricity. Further experiments are to be made 
with receivers of very great capacity. 
b. Preliminary Experiments on the Thermal Conductivity of some 
Dielectrics. By Messrs C. M. Smith and C. Gr. Knott. 
These experiments were suggested by observations on the different 
lengths of time required, under different circumstances, for telegraph 
cables to assume the temperature of the water in which they were 
submerged. The method employed was that known as “ Ang- 
strom’s,” which has already been described by Prof. Tait (Proc. 
B. S. E. 1872-73 p. 55-61); the manner of the application of the 
method being, however, somewhat modified in these experiments, 
we will give a short description of it. 
