Surron—Influence of Water-Vapour upon Nocturnal Radiation. 17 
chilling, almost a year before he published the results which 
Very? criticises; and hence it may be taken for granted, I think, 
that Tyndall intended it to be understood that the temperature of 
the vapour was the same at each pressure. It is not very likely, 
judging from Tyndall’s record, that any objections to his results 
can be maintained on the ground of faulty manipulation. 
In this connexion one of Boyle’s observations is worth quoting 
here by the way for the sake of its historic interest. After what 
Tyndall has said with reference to his own experiment with the 
air-pump, the explanation of the phenomenon (which Boyle does 
not give) will be obvious :— 
“We have observ’d,” says Boyle, “ That though we convey ’d 
into the Receiver our Scales, and the Pendu/a [of iron or steel] 
formerly mention’d, clean and bright; yet after the Receiver had 
been empty’d, and the Air let in again, the gloss or lustre both of 
the one, and of the other, appear’d tarnish’d by a beginning 
rust.’’? 
Returning again to Tyndall, it seems to me not at all improb- 
able that six times as much vapour in the given space did actually 
exert about six times as much absorption. But while, with all the 
respect which is due to Tyndall’s great powers as an experi- 
mentalist, it may be suggested that such a result is just as likely 
to be due to the resulting relative humidity as to the actual 
quantity of vapour present, there is a further objection that the 
meteorological problem is not necessarily so simple a case as he 
supposed. The air, according to his own showing, absorbs and 
radiates heat. It absorbs its own radiations as well as those from 
the surface of the Harth. Considering a small thickness of the 
lower air, we may be sure from Tyndall’s own results (as indeed 
he has carefully pointed out) that the drier it is the less heat it 
will absorb, and therefore, once it is heated, the more slowly it will 
cool of itself. On the other hand, the surface of the ground should 
1J. Tyndall, ‘On the Absorption and Radiation of Heat by Gaseous Matter,” 
Phil. Trans., 1862, p. 59. 
2 Robert Boyle, ‘‘New Experiments,” 1682, p. 161. Expt. xxxvil. and XXXIx. 
should also be consulted. 
