Please tell me why does Holmes want to drain the moat?

Holmes wants the moat drained to see if it holds any evidence of the murder.

“It has struck me that it is our duty to drain the moat, in the hope that we may find some—”

“It's impossible,” said the inspector. “I've made inquiry.”

“Tut, tut! My dear sir, please do what I ask you.”

“Well, go on.”

“—in the hope that we may find something which may bear upon our investigation. I have made arrangements, and the workmen will be at work early to-morrow morning diverting the stream—”