The second is trapping the neutrons in a metal bottle, with magnetic fields or
even via gravity, similar to how water is «trapped» in a bathtub.
In fact, at the time of Maxwell's death, his theory of electromagnetic fields was not yet widely accepted or
even well known; experts still argued about whether electricity and magnetism propagated their effects
via «action at a distance,» as
gravity (supposedly) did, or by Michael Faraday's «lines of force» (incorporated by Maxwell into his fields).