The Monster Men by Edgar Rice Burroughs

One day, about two weeks later, von Horn and the professor were occupied closely with their work in the court of mystery. Developments were coming in riotous confusion. A recent startling discovery bade fare to simplify and expedite the work far beyond the fondest dreams of the scientist.

Von Horn's interest in the marvellous results that had been obtained was little short of the professor's-but he foresaw a very different outcome of it all, and by day never moved without a gun at either hip, and by night both of them were beside him.