Axel outlines in detail the various gear the travelers will take with them to the center of the earth, dividing it into categories: instruments to take measurements and guide the journey; arms (guns); tools, such as ropes and crowbars; provisions (food: Axel notes with alarm they bring no water under the assumption it will be found below the earth), and medical supplies. Axel doesn't fail to mention the tobacco his uncle carries with him, a pollutant that would be forbidden today.
Two pieces of gear Axel spends some time describing include Ruhmkorff's coil. This is an electrical mechanism that will shed light, allowing the explorers to see as they travel to the center of the earth. Ruhmkorff's coil consists of copper wire, insulated with silk, surrounded by another insulated coil. The wires generate a current from a voltaic battery which will produce a continuous white light. The coils will be carried in a leather bag with a shoulder strap. This would have been state-of-the-art technology in the nineteenth century, an early version of a flashlight that we would today take for granted.
The explorers also take with them a centigrade thermometer. This instrument can measure up to 150 degrees. Axel notes that if the temperature gets that high, they will be "cooked." On the other hand, it is not enough to measure the exact temperature "of springs or metal in a state of fusion."
Axel shows the forethought that goes into the gear choices but also introduces enough anxiety about the supplies—or lack thereof—to create some tension in the reader's mind.