298 SECTIONAL TRANSACTIONS.—A. 
Friday, August 28. 
8. Presidential Address by Dr. G. C. Stueson, F.R.S., on The New 
Ideas in Meteorology. (See page 15.) 
9. Sir Frank Dyson, F.R.S.—Fiaing the Position of the Equator. 
10. Sir Narrer Suaw, F.R.S.—Trigger Action in the Atmosphere. 
In discussions of the transformations of energy in the atmosphere it is sometimes 
suggested in explanation of certain kinds of rainfall that there may be something in 
the atmosphere analogous to catalytic action by which the transformation is initiated 
and continued, although the energy which is transformed is not supplied by the 
initiating agent, but is derived from the interaction of the constituent parts of the rest 
of the atmosphere. One of the characteristic features of such a condition is the dis- 
continuity between two states which differ only in the presence or absence of the 
‘ catalyser’ or initiating agent. 
A certain line of real discontinuity in atmospheric conditions is marked by the 
saturation of the air with water-vapour, the behaviour under the gradual reduction 
of pressure being fundamentally different when the air is saturated from what it is 
when unsaturated. Thus the consideration of saturated air in the atmosphere requires 
the introduction of ideas more or less suggestive of ‘ trigger action,’ and the purpose 
of this communication is to trace the effect of saturating air with water-vapour as an 
example of ‘ trigger action.’ 
Assuming that the condensation of water-vapour to form rain is a consequence of 
the reduction of pressure, we recognise at the outset that the transformation of energy 
associated with rain is two-fold in character. There is first the gravitational energy of 
the environment which forces some portion of the air to rise, and secondly there is the 
development, in the form of sensible heat during elevation, of what was originally 
latent in the vapour. The energy of the second kind operates to limit the reduction of 
temperature, by reduced pressure, to an amount much below that which would be 
consequent upon the same reduction of pressure if the air were dry, and thereby to 
maintain or even increase the amount of energy available in the environment for 
further automatic elevation. 
The development of ancillary energy in the environment in this way may be so 
great as to place the originally saturated air in a condition which, without unfairness, 
may be described as explosive; and it is on that account that the name ‘ trigger 
action * may be applied to it. 
But although in this way saturated air may be regarded as a trigger which when 
properly operative sets the whole transformation in motion or even in commotion, yet 
the ancillary energy comes from the saturated air itself and the supply is limited by 
the amount of saturated air available. In a sense therefore the trigger has to expend 
lee in producing the explosion, and the process is on that account not exactly 
catalytic. 
The communication draws attention td the parts which are taken, by the environ- 
ment and the environed air respectively, in this explosive action, and illustrates the 
subject by examples derived from the exploration of the pressure, temperature and 
humidity of the air by sounding balloons, kites or aeroplanes. For this purpose the 
results of soundings are set out in certain new kinds of diagrams which, for the sake of 
reference, are called ‘tephigrams’ and ‘ depegrams.’ A tephigram, like an indicator 
diagram, shows the properties of the environing air in a vertical section of the atmo- 
sphere referred to (¢) temperature and ($) entropy as co-ordinates, with a background 
exhibiting the physical properties of saturated air, and a depegram shows the properties 
of the air in the same vertical section referred to the temperature of its dewpoint (d) 
and its pressure (p). 
11. Sir Girgert WaLker, F.R.S.—Seasonal Variations of Weather in 
the North Atlantic. 
Of the three well-established oscillations—that in the southern oceans, that in 
the N. Pacific, and that in the N. Atlantic—the last is largely independent of the 
an 
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