1. Humboldt and Bonpland were on horseback.
    洪堡和庞普朗骑在马背上，

2. Fortunately for me, as I was wondering what to do next, there appeared on the horizon a man on horseback, riding in my direction.
    不过，我是幸运的，正当我走投无路之时，从天边出现一个骑马的人向我骑来。