14 Scientific Proceedings, Royal Dublin Society. 
with observation is almost perfect.”! And Tyndall went on 
to say :—‘“The power of aqueous vapour being thus established, 
meteorologists may, I think, accept the result without fear. That 
10 per cent. of the entire terrestrial radiation is absorbed by the 
aqueous vapour which exists within ten feet of the Harth’s surface 
on a day of average humidity, is a moderate estimate. In warm 
weather, and air approaching to saturation, the absorption would 
probably be considerably greater. ‘This single fact at once sug- 
gests the importance of the established action as regards meteoro- 
logy. Iam persuaded that by means of it many difficulties will 
be solved, and many familiar effects which we pass over without 
sufficient scrutiny because they are familiar, will have a novel 
interest attached to them by their connexion with the action of 
aqueous vapour on radiant heat.’” 
There seems to be no doubt that Tyndall’s experiments were 
carefully carried out, and that the observed effects were not vitiated 
by any extraneous disturbances. But the point which should 
immediately appeal to a meteorologist, and which Tyndall (not 
being a meteorologist) seems to have overlooked, is that humid air 
has always to be considered in two ways, namely, from the point 
of view of both relative and absolute humidity. By doubling the 
quantity of vapour in a given space, so long as the temperature 
is unaltered and no condensation takes place, the humidity is 
doubled as well as the tension. By doubling the space, the tem- 
perature being kept constant, the tension and humidity would both 
have been halved. If Tyndall had made this latter variation of 
his experiment, and the absorption had remained the same, then 
undoubtedly he could have claimed that it was proportional to the 
quantity of aqueous vapour present. He did not make the varia- 
tion, however; therefore, so far as his table of absorption is con- 
cerned, there is nothing to show that the observed effect might 
not have been just as much due to the relative as to the absolute 
humidity.® 
1 J, Tyndall, ‘‘ On the Relation of Radiant Heat to Aqueous Vapour,”’ Phil. Trans., 
vol. cliti., 18638, p. 1. 
2 Tbid., p. 8. 
3 Tyndall did show that a small quantity of ether vapour, under a given pressure, 
would exert the same absorption upon the radiations from lime-light as when it was 
