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Lincoln's Secret Weapon

Classroom Activity

To explore how principles of gas behavior relate to diving in order to plan safe underwater activities.

Materials for each team
  • copy of student handouts
    Dive, Dive, Dive ( HTML)
    Let's Go Diving ( HTML)
    Planning Your Profile ( HTML)
  • paper and pencil
Materials for teacher
  • 2 bottles of sugarless club soda, 12 ounce or liter, screw cap
  • kitchen pan
  1. You can demonstrate the need for decompression stops with two warm bottles of club soda. First, shake the two warm bottles. Holding the first bottle over the kitchen pan, slowly unscrew the cap of one bottle until small fizz bubbles appear and some gas escapes. Tighten the cap and explain that this "diver" is now at a decompression stop and must wait until the extra gas has been released. Slowly unscrew and retighten the cap until all the gas has escaped.

  2. Hold the second bottle over the kitchen pan. Explain that it represents a diver that came up with no stops. Unscrew the cap completely in one motion. All the gas that was in solution in the higher-pressure bottle comes out in one fast action.

  3. Organize students into teams of two and distribute copies of the "Dive, Dive, Dive," the "Let's Go Diving," and the "Planning Your Profile" student handouts to each team.

  4. Explain to students that they are going on a diving vacation and must plan their dive schedules, or profiles, before they ever get in the water.

  5. Tell students that they will be calculating for short, medium, and long dives. Explain that in a real situation, divers factor estimated and actual time at depth. Also explain the activity uses a fixed degassing rate to represent the idea that divers degas while on the surface. Actual degassing rates are more complicated.

  6. Conclude with a discussion about how these principles of gas behavior were a consideration in the program. Talk with students about the differences between their dive activity (where their dives were shallow enough—less than 100 feet (30.5 meters)—to use compressed air only) and the dives done in the program (which were so deep that they required divers to breathe a special gas mixture to help combat the narcotic effects of nitrogen at certain depths).

Activity Answer

The amount of atmospheric gases that dissolve in human blood and tissue depends on the surrounding pressure. As divers descend, the increasing pressure causes ever-greater amounts of nitrogen gas to dissolve in their blood. As they return to the surface, this extra dissolved gas will leave the body through the lungs where it is removed from the blood and released; but only if they ascend slowly—if they don't, large nitrogen bubbles could form causing decompression sickness (also known as the bends). Divers control the release of gases from their blood by ascending in stages called decompression stops. These stops reduce the pressure slowly, allowing gas to escape so that large bubbles do not form.

Students will discover that there are several safe dive profiles each day. Here's an example:

Dive Profile

First dive, wreck, medium time, 80 feet

+12 points

Rest for one hour

- 1 point

Second dive, Ray habitat, medium time, 35 feet (40 feet)

+ 3 point

Rest for three hours

-3 points

Last dive, search for lobsters, long time, 20 feet (40 feet)

+ 12 points

Total dissolved gas points

23 points

The lower the day's dive points, the safer the dive profile. Note that the third day should be reserved for exploring and shopping because no diving is allowed within 24 hours of flight. (See guideline No. 4.)

Scanning the rows and columns of the dive table shows that the longest dives earn more points. Similarly, the deepest dives also earn more points.

In the program, once the divers began to cut the propeller shaft, it became clear that the operation would take more time than originally thought. The question then became which would come first: The cut would be completed, or all divers would reach their maximum dissolved gas levels and have to leave the job unfinished. An even more important factor for them to consider was the environmental conditions they faced, such as strength of underwater currents and surface weather conditions.

Important Note
Diving can be a dangerous sport, which is why it's one of the few recreational activities that certifies participants. The Diving Table on page 8 is loosely based on dive tables used by the U.S. Navy without decompression stops and is included here for the purpose of introducing the basic concept of diving physiology. Its utility is limited to this purpose only. Potential divers must receive proper instruction by enrolling in a diver training program offered by recognized certification agencies.

Links and Books


Davis, William C. Duel Between the First Ironclads. Mechanicsburg, Pennsylvania: Stackpole Books, 1994.
Presents the history of the building of the Monitor, combining the very latest in naval invention and technology.

De Kay, James T. Monitor: The Story of the Legendary Civil War Ironclad and the Man Whose Invention Changed the Course of History. New York: Walker, 1997.
Focuses on how ironclad ships came into being, including John Ericsson's work of over 20 years to have his vision of a warship become reality.

Mindell, David A. War, Technology, and Experience Aboard the USS Monitor. Baltimore, Maryland: Johns Hopkins University Press, 2000.
A carefully documented history that reads like an adventure story. It examines the experience of the Monitor's crew and its reactions to the thrills and dangers that came with this new war technology.


Grim, Pamela. "Trouble on Flight 1368." Discover, September 2000, 46.
Details the case of a woman who experienced decompression sickness during a flight she took following a diving vacation in Bimini.

Web Sites

NOVA Online—Lincoln's Secret Weapon
Follows the story of the futuristic armored combat vessel, Monitor, which opened up a new chapter in naval warfare. Reveals more about the Navy's risky efforts to salvage the secrets of the Monitor, and includes articles, activities, resource links, and more.

Diving Medicine Online
Contains myriad information about the physiological effects of diving.


The "Dive, Dive, Dive," "Let's Go Diving," and "Planning Your Profile" activities align with the following National Science Education Standards:

Grades 5-8

Science in Personal and Social Perspectives

Science Standard F:
Science in Personal and Social Perspectives

Personal health

  • The potential for accidents and the existence of hazards imposes the need for injury prevention. Safe living involves the development and use of safety precautions and the recognition of risk in personal decisions. Injury prevention has personal and social dimensions.

Grades 9-12

Science in Personal and Social Perspectives

Science Standard F:
Science in Personal and Social Perspectives

Personal and community health

  • Hazards and the potential for accidents exist. Regardless of the environment, the possibility of injury, illness, disability, or death may be present. Humans have a variety of mechanisms—sensory, motor, emotional, social, and technological—that can reduce and modify hazards.

Teacher's Guide
Lincoln's Secret Weapon

Video is not required for this activity