Patricia Wentworth 
The Benevent Treasure [EPUB ebook] 

The ledge was about six inches wide. Candida stood on it with her toes stubbed against the rock. Her left hand was clenched on a small projecting knob about level with the top of her head. With the other she was feeling carefully and methodically for something which she could catch hold of on her right. There didn’t seem to be anything, but she went on feeling. In the end she had to come back to the shallow crack which she had discarded. It would only take the tips of her fingers. By itself it really wasn’t any good, but it did just give the least little help to the hand that was clutching the knob. She stood there and wondered what she was going to do next.