170 TRANSACTIONS OF THE J APR. 22, 
a kind of shelly deposit, and they bore onward as they increase 
in size. The ordinary Zeredo navalis does not grow to be very 
large, but in the waters off the coasts of the American isthmus 
ship-worms become sometimes ten inches or even much more in 
length. Many a gallant vessel has been obliged to succumb to 
these creatures, the hull becoming honeycombed with the holes 
which they have bored. It has been supposed that if ships could 
go suddenly from salt water into fresh water, or lie at anchor 
in a large fresh-water lake,—such as Lake Nicaragua,—they 
would be delivered from these pests; but the dreaded Teredo 
has been found in brackish water, and one distinguished natu- 
ralist has claimed that it has been found alive even in fresh 
water. But could a vessel be taken out of the water,—as when 
it is being transported across the isthmus of 'Tehuantepec,—it is 
almost certain that the ship-worms would quickly die, as they are 
so sensitive to air that they never attack piers or vessels above the 
water-line. 
Another foe to ships,—of iron as well as of wood,—is the little 
crustacean shell-fish commonly known as the barnacle. These 
fasten themselves in such numbers on a vessel that its speed be- 
comes much affected. Fast steamers,—even the finest which sail 
the seas,—must be taken out of the water from time to time, to 
rid them of these pests. The barnacles, it is believed, could 
not live if a vessel could enter fresh water, or lie at anchor in a 
lake; but though the barnacles would die, their shells would be 
so cemented to the vessel that it would be needful for the hull 
to be taken out of the water and scraped. Lads, in planning 
his ship-railway, made provision for side-tracks upon which the 
ship-car can be run while the ship with which it is loaded is 
scraped and painted, and refitted in every way. 
Tehuantepec is some six or seven hundred miles nearer to 
the United States by land than is Nicaragua. By looking at a 
map it can be seen that, while Panama and Nicaragua are bathed 
by the waters of the Caribbean Sea, Tehuantepec is washed by 
the waters of the almost land-inclosed Gulf of Mexico. This 
great American gulf is about one thousand miles in length by 
eight hundred in width, and receives the discharge of a vaster 
river system than is found on any other continent. Into it emp- 
ties the Mississippi, which, with its forty-two navigable tribu- 
taries, waters in part twenty-one States and Territories. Four- 
teen of these are sometimes called ‘‘ Mississippi Valley States.” 
This vast, fair, and fertile region is capable of supporting hun- 
dreds of millionsof people. Joined,—or to be joined,—with the 
Mississippi River by a canal, and possibly by a railway for vessels, 
are the great American lakes,—inland seas,—which have been 
estimated to contain about one-third of all the fresh water in 
