How does Stapleton use the hound to try to murder Sir Henry?
The stratagems that Stapleton devises to murder Sir Charles and Sir Henry by means of the hound have two purposes. First, by reinforcing the legend of the hound, Stapleton hopes to divert attention from himself and escape punishment. Second, this appears to Stapleton an effective way of removing obstacles to inheriting the Baskerville fortune.
Stapleton's first step is acquiring a huge black dog, the ancestry of which is a mix of mastiff and bloodhound. This dog resembles the hound of the legend. Watson describes it as follows after its death:
In mere size and strength it was a terrible creature which was lying stretched before us. It was not a pure bloodhound and it was not a pure mastiff; but it appeared to be a combination of the two—gaunt, savage, and as large as a small lioness. ... The huge jaws seemed to be dripping with a bluish flame and the small, deep-set, cruel eyes were ringed with fire.
The glow on the hound's body and head is actually phosphorus, a preparation applied to it by Stapleton to give the hound an eerie supernatural appearance.
Stapleton steals one of Sir Henry's boots to train the hound to be attracted to Sir Henry's scent and then starves the hound to make it more vicious. He then tries to lure Sir Henry on to the moor to be attacked, but is thwarted by Holmes.