Can you solve this 1932 brain teaser concerning two vehicles?

ballorwaterIn the early part of the 1900’s, automakers were still experimenting with fuel sources – and the basic laws of physics. The pic to the right is a riddle from a 1932 edition of Modern Mechanix magazine where they asked readers to figure out which vehicle would be propelled forward to send in their answer. $10 would be handed out to a random correct entry. $10! Gee whiz, mister. Spanks a bundle. So yeah, which truck would move forward, genius?

Modern Mechanix via MAKE