Periscopes and diesel engines

Around this time, two other improvements were introduced. Inventor
Simon Lake created the first periscope specifically for submarines. A
periscope is a vertical telescope that provides a magnified view and a wide
angle of vision. In the 1890s, Rudolf Diesel invented an engine that was
fired by compression rather than an electric spark. The diesel engine was
more economical than the gasoline engine and its fumes were much less
toxic (poisonous) and volatile (explosive). This new engine became essential
to all submarines until nuclear power was introduced as a means
of propulsion in the 1950s.
In World War II (1939–45), submarines played a large role in Germany’s
repeated attacks on Allied (English, American, and French) ships.
Meanwhile, American submarines crippled the Japanese by sinking nearly
1,400 merchant and naval ships. During this time, the snorkel was developed.
It was a set of two fixed air pipes that projected from the sub’s
topside. One tube brought fresh air into the vessel, and the other vented
engine exhaust fumes. Now a sub could stay hidden below the surface
when running on its diesel engine and recharging its batteries.