PRESIDENTIAL ADDRESS. 176 
(a) volcanic action, (b) formation and melting of hoar 
frost, (ce) vegetation. The first class is illustrated most 
forcibly by the crater Linné, which according to 
Lohrmann, Madler and Schmidt, prior to 1848, had a 
diameter of between four and seven miles, whereas its 
present diameter is three-quarters of a mile. A new 
crater has also been announced in the vicinity of 
Hyginus. The floor of the crater Plato has repeatedly 
furnished new formations at various times—Pickering’s 
latest observations show the existence of a crescent- 
shaped bank, six miles long by one or two miles broad, 
not shown on maps made by A. 8S. Williams during the 
period of 1879-1890. Turning to the second class of 
changes, there appears to be numerous examples of 
formations which would be most simply explained by 
supposing them to be composed of hoar-frost. It is 
considered that the strongest evidence that water in the 
liquid state ever existed upon the surface of the moon 
lies in the dried up river-beds of which the best example 
lies on the eastern slopes of Mount Hadley, at the base 
of the Appenines. Another, discovered only in the 
summer of 1904, lies about 60 miles due south of Conon. 
The author also brings direct observational evidence to 
show that changes in the forms of lunar features e.g. the 
small craters Messier and Messier (a)-may be actually 
seen to vary with the period of the lunar day. 
Regarding the third class of variable phenomena, these 
are stated to be more conspicuous than those due to the 
former two causes. 
Reproductions are given in the paper of four photographs 
obtained in Jamaica in 1901, showing distinct evidence of 
changes on the crater Hratosthenes with the age of the 
moon. Hvery precaution has been taken to exclude any 
conclusions based on changes of shadow resulting from 
B—May 2, 1906. 
