Jules Verne is nothing if not methodical in his listing of the equipment and supplies for the expedition. In addition to the mineralogical articles (mentioned during the build up to the journey, and presumably regulation stuff for sedimentary analysis) and medical kit, the party carries “numerous packages.”
There are some standard pieces for rock climbing and spelunking:
Hatchet, hammer, your general blunt instruments
A ladder, rope and pitons
The more specialized instruments are as follows:
An Engel’s centigrade thermometer
A manometer (to detect atmospheric pressure at levels above sea level; an ordinary barometer wouldn’t do below the earth’s surface)
A chronometer (set to the Hamburg meridian)
Note: There’s one chronometer listed. The outline of tools may be general instead of specific; at one point in the narration, Axel and the Professor apparently each have their own chronometer, helping them track each other.
A “night glass”
And the Ruhmkorff’s apparatus, essentially a 19th century flashlight.
There are also firearms, which give the somewhat hesitant Axel pause. Will they be needing weapons?