DDT: In the late 1930s, with war on the horizon, the U.S. military undertook preparations to defend soldiers against one of the most lethal enemies on the battlefield: insect-borne diseases. During World War I, typhus—a bacterial disease spread by lice—had killed 2.5 million people (military and civilian) at the eastern front alone. Health specialists also worried about the prospect of mosquito-borne diseases, such as yellow fever and malaria, in the tropics.
The military needed an insecticide that could be safely applied as a powder to clothes and blankets. Initially synthesized by an Austrian student in 1873, DDT (dichlorodiphenyltrichloroethane) remained a laboratory oddity until 1939, when Swiss chemist Paul Müller discovered its insecticidal properties while researching ways to mothproof wool clothing. After the military screened thousands of chemical compounds, DDT eventually emerged as the insecticide of choice: it worked at low dosages, it worked immediately and it kept working.
DDT proved to be so effective that some historians believe World War II was the first conflict where more soldiers died in combat than from disease. Yet, even before the war ended, entomologists and medical researchers warned that the insecticide could have long-term, dangerous effects on public health and the environment. The United States banned DDT in 1972.
Tide-Predicting Machine: As the Allies planned their invasion of Europe in 1944, they faced a dilemma: Should they land on the beaches of Normandy at high tide or low tide?
The argument in favor of high tide was that troops would have less terrain to cross as they were subjected to enemy fire. However, German Gen. Erwin Rommel had spent months overseeing the construction of obstacles and booby traps—which he called a “devil’s garden”—to thwart a potential Allied landing. During high tide, the devil’s garden would be submerged and virtually invisible; but during low tide it would be exposed.
Ultimately, military planners concluded that the best conditions for an invasion would be a day with an early-morning (but steadily rising) low tide. That way, landing craft could avoid the German obstacles, and Army engineers could begin clearing them away for subsequent landings.
To complicate matters, the Allies also wanted a date when, prior to the dawn invasion, there would be sufficient moonlight to aid pilots in landing paratroopers.
So the Allies consulted meteorologists and other experts to calculate the dates when the tides and the moon would meet the ideal conditions. Among those experts was Arthur Thomas Doodson, a British mathematician who had constructed one of the world’s most precise tide-predicting machines—which reduced the risk of ships running aground when entering a harbor. Doodson’s machine was essentially a primitive computer that produced calculations using dozens of pulley wheels. Doodson himself calculated the ideal dates for the D-Day invasion—a narrow set of options that included June 5-7, 1944. The Allied invasion of Europe commenced on June 6.