In the novel Ordeal by Innocence, Dr Arthur Calgary is a geophysicist who gave a lift to Jacko Argyle two years earlier, and then was out of the country when Argyle was on trial for murder. Thus Calgary could not give his testimony, which would have backed up Argyle's statement. He does give it when he returns to England and learns what happened. He was a member of the Hayes Bentley expedition to the Antarctic.
Dr Calgary has some friends in Redquay.
In the Ordeal by Innocence BBC miniseries, Dr Calgary was portrayed by Luke Treadaway. Here he claims to have gone to a trip to the Arctic but it turns out he is a patient of a mental hospital. He had escaped in the evening when Rachel Argyle was killed, and could give an alibi to Jacko Argyle. In this version Jacko isn't the murderer but it's his father Leo.