Why was Sir Henry upset by the mysterious cry he heard the night he and Dr. Watson went out to catch the prisoner? (chapter 7-9)

Sir Henry recognized the sound as what the people called the cry of the Hound of the Baskervilles.

‘Watson,’ said the baronet, ‘it was the cry of a hound.’

My blood ran cold in my veins, for there was a break in his voice which told of the sudden horror which had seized him.

‘What do they call this sound?’ he asked.

‘Who?’

‘The folk on the country-side.’

‘Oh, they are ignorant people. Why should you mind what they call it?’

‘Tell me, Watson. What do they say of it?’

I hesitated but could not escape the question. ‘They say it is the cry of the Hound of the Baskervilles.’