Warships (five): WATCHING television is an education in itself these days.

Every night there's a documentary telling you everything you never wanted to know about a fresh topic.

At least Major Gordon Corrigan, our guide through the history of the submarine, was friendlier than the fierce teachers on Channel 4's That'll Teach 'Em, in which modern day teenagers are subjected to school 1950s-style.

In the latest episode, the boys were told by their teacher there was to be no "beastliness" in the dormitory after matron's discovery of what was delicately described as "sticky sheets". I don't think this was to do with eating Spangles under them.

From Major Corrigan, we learnt about the considerable impact of submarines on naval warfare because they're based on the simple premise of surprise. They approach the enemy unseen, attack, and slip away again undetected - "the ultimate stealth weapon". Today, nowhere is out of reach of the submarine, and they carry enough weaponry to destroy a continent.

Nuclear-powered submarines, which can travel faster and stay underwater longer than ever before, are positively luxurious compared with early versions, although they are still not recommended for claustrophobics or those who find alarming the thought of travelling under the Channel by train.

How different to the first submersible used during the American Revolution, a one-man, hand-powered barrel. The plan was to plant an explosive device on a British flagship in New York Harbour. Unfortunately, operator Ezra Lee was forced to surface and was spotted. The mission was a failure, but he managed to get away.

Eighty-eight years later, a sub called a David took to the water in the American Civil War. This 60ft long craft, carrying a captain and eight men, could only just get under the surface and its weaponry was primitive - an explosive charge on the end of an 18ft pole extending from the front of the sub.

The snag was the craft couldn't get away fast enough from the ship it was sinking, and went down too. It took the invention of the torpedo to enable strikes from a distance.

By the end of the 19th century, the idea of a fully submersible craft was being realised. Not without a few false starts, such as the steam-driven sub where the heat inside became intolerable once it was fired up.

Finally, in 1897, the notion of a sub driven by battery-power established the key principles on which all future subs would be built.

These subs changed the rules of war. Before, enemy boats were stopped, boarded, papers inspected, and everyone given a chance to get away before the captured vessel was sunk. U-boats could sneak up and destroy without warning.

Modern subs can hit targets on land too. They are "the ultimate invisible means of delivering massive destructive forces" said the Major. All very impressive, but he never did tell us what sailors do about sticky sheets.