The Atlantic is running a great series on World War One in pictures. One of the fascinating things you see in these pictures is the fascinating mix of old and new technology.
For instance, these American soldiers are using an acoustic locator to find faraway aircraft. The horns collect faint sounds, like the sound of an airplane engine. An operator listens on a headset, moving the direction of the apparatus. The distant sounds become louder when the sound receivers are more directly pointed at the source of the noise, and so by moving the apparatus until the sound it as its loudest the operator can locate the source of the noise. This represented the best technology for locating too-far-to-see aircraft up until the invention of radar during World War II.
The military use of radio was in its infancy during the war. Here two members of a German communications squad power a radio transmitter by pedaling a tandem bicycle generator.
Airplane cockpits were still open, causing aviators to struggle against wind as well as cold. This German pilot is wearing a mask and heated suit.
Poison gas is one of the most famous evils of the war. The horror of it was such that it was banned by international convention after the war and has not been used in major wars between great powers since. Here, German storm troops move through a cloud of poison gas.
Bicycle troops were expected to be a key means of exploiting modern technology to cover distance quickly, though they ended up being of limited use once static warfare set in. These are Belgian bicycle troops at the beginning of the war.
Airplanes were increasingly used for observation, but it was difficult to get real-time communication from them. Radios were bulky for the small, light airplanes of the time. As a result artillery observation (which required immediate observation and feedback to the gunners) was often carried out via static balloons such as this one. They were tethered with cables that allowed them to be raised and lowered. One or more observers would get into the gondola and then use a field telephone (also wired) to call down observations. Stationary balloons, however, made a great target for airplanes, and one of the precursors for a major attack was often to shoot down all the enemy observation balloons, thus reducing their ability to spot troop movements and call in artillery direction. In this photo you can see a motorcycle soldier in the foreground, and an observation balloon behind.
Machine guns did horrible execution during the war (though it was artillery which truly ruled the battlefields). During the latter half of the war light machine guns became more common -- not light in an objective since, as they still weighted fifteen to thirty pounds, but light in the sense that they could be carried on the move by one man. However, the primary workhorse remained the tripod-mounted, water-cooled .30 machine gun. These could weigh upwards of a hundred pounds and thus there was the problem of how to move them around. The mixture of solutions underlines the strange mix of old and modern which characterizes the period: