LIMITS OF HUMAN POWER. 47 
ed, could human cunning rescue its wasted hillsides and its 
deserted plains from solitude or mere nomade occupation, from 
‘barrenness, from nakedness, and from insalubrity, and restore 
the ancient fertility and healthfulness of the Etruscan sea coast, 
the Campagna and the Pontine marshes, of Calabria, of Sicily, 
of the Peloponnesus and insular and continental Greece, of 
Asia Minor, of the slopes of Lebanon and Hermon, of Pales- 
tine, of the Syrian desert, of Mesopotamia and the delta of the 
Euphrates, of the Cyrenaica, of Africa proper, Numidia, and 
Mauritania, the thronging millions of Europe might still find 
room on the Eastern continent, and the main current of emi- 
gration be turned towards the rising instead of the setting sun. 
But changes like these must await not only great political 
and moral revolutions in the governments and peoples by whom 
those regions are now possessed, but, especially, a command of 
pecuniary and of mechanical means not at present enjoyed by 
those nations, and a more advanced and generally diffused 
knowledge of the processes by which the amelioration of soil 
and climate is possible than now anywhere exists. Until such 
circumstances shall conspire to favor the work of geographical 
regeneration, the countries I have mentioned, with here and 
there a local exception, will continue to sink into yet deeper 
desolation, and in the meantime the American continent, 
Southern Africa, Australia, New Zealand, and the smaller 
oceanic islands, will be almost the only theatres where man is 
engaged, on a great scale, in transforming the face of nature. 
course, needed little fire to bring it to boil. But this was the direct and sim- 
ple, not the concentrated or accumulated heat of the sun. 
On the utilizing of the solar heat, simply as heat, see the work of Moucnor, 
La Chaleur solaire et ses applications industrielles. Paris, 1869. 
The reciprocal convertibility of the natural forces has suggested the possi- 
bility of advantageously converting the heat of the sun into mechanical power. 
Ericsson calculates that in all latitudes between the equator and 45°, a hun- 
dred square feet of surface exposed to the solar rays develop continuously, for 
nine hours a day on an average, eight and one fifth horse-power. 
I do not know that any attempts have been made to accumulate and store 
up, for use at pleasure, force derived from this powerful source. 
