240 
in the application of the Rule of Three te geological chro- 
nology),—or whether the transporting force of a stream was 
greater than, less than, or equal to its denuding force: each 
case had to be judged by itself. In many cases he thought that 
at the present day the taluses were increasing on the cirques ; 
that, however, was only a question of rainfall, strata, and the 
like: further it must not be forgotten that there is chemical as 
well as mechanical denudation. 
The Master of EMMANUEL asked whether there was an ob- 
served difference of constitution in the rocks to explain either 
the floor or a difference in the slope in the walls of the cirque. 
Mr Bonney replied that it was in the case of most cirques 
difficult to say positively, seeing that their floors were often 
masked by talus, vegetation, &c., and that in the case of the one 
described in the paper, in the Syenite, there was no distinction 
that was conspicuous, although there very probably was some 
in the chemical constitution of the rock. Differences in the 
slope of the walls of a cirque in sedimentary rocks were doubt- 
less due to difference in the strata. 
Some further conversation took place in which Mr Potter, 
Mr O. Fisher, and others took part. 
November 13, 1871. 
The PRESIDENT (PROFESSOR HUMPHRY) in the Chair. 
Communications were made to the Society : 
(t) On a double action Pentagraph. By Prof. Carty, 
E.R.S. 
The machine was exhibited and described. 
(2) On the experimental verification of the laws of the 
resistances which bodies are subject to, in moving 
through the air; and especially on the experrments 
made by Mr Robins and Dr Hutton with the 
Whirling Machine. By R. Porrzr, M.A. 
