ANNIVERSARY ADDRESS. 35 
matter to calculate exactly how much powder or fuel would be 
required to effect the desired result. Some of these conditions, 
however, especially the relative temperatures of the several layers 
of the atmosphere, cannot be ascertained, and others are exceed- 
ingly difficult to measure. We may, however, get some idea of 
the amount of force necessary by studying certain phenomena of 
rainfall which are presented to us in various parts of the earth. 
Many illustrations of the fall of rain from ascending currents 
might be selected ; but the following will suffice for our present 
purpose. The island of Port Rico in the West Indies extends 90 
miles east and west, and only 30 miles north and south; a chain 
of mountains, from 1,500 to 3,700 feet high, extends along the 
island from east to west. Throughout the year the N.E. trade 
wind blows on to the island every day from 9 a.m. to sunset, and 
at night there is a strong land breeze toward the ocean on all 
sides. During the rainy season, that is from the end of May 
the end of October, the rain falls every day on the northern 
portions of the island from 2 p.m. to sunset. This is due to the 
mountains, which turn up the trade winds, saturated as they are 
with vapour in the afternoon, into the colder regions, and thus 
eause precipitation of rain. But this is all on the northern slope ; for 
on the south side not a drop falls from this wind, and sometimes 
this part of the island suffers from drought for more than a year 
without interruption. So well known and so constant is this con- 
_ dition that it is proposed to’ tunnel through the mountains, and 
_ thus bring some of the superabundant waters of the north to the 
South side for the purpose of irrigation. It is instructive to notice 
here what & very moderate rise will cause constant rain from a 
Wind that is nearly saturated with moisture; but I am sorry I 
_ have not got observations of the actual state of the air, and the 
— the mountains in quantity of rain. , 
Again, the celebrated rainy spot in India where the annual 
at is counted by hundreds of inches is a place of exactly — 
