WHY did he blast to death a fellow guard? He was unhinged, said a doctor. A magistrates clerk agreed but a judge was less convinced. A jury had to decide on his motive.

The drama happened on September 2, 1842, when William Davis and Stephen Clasby were patrolling fields near Haslar Hospital in Gosport keeping watch on a prison working party from the convict ship York moored in the town's harbour.

Suddenly, a shot rang out and fellow guard Robert Beasley saw Clasby fall to the ground just a foot away from Davis.

"I ran to the spot about 40 yards away and found Mr Clasby dead," he recounted to jurors. "I went up to Davis and asked him who authorised him to shoot him. He said he didn't do it, he did it himself. I took the musket away from him and found it had been recently discharged. There were no convicts near where Mr Clasby fell."

Defence counsel Mr Sewell suggested it had might have been a case of mistaken identity but Beasley refuted it. "Mr Clasby was not dressed like a convict."

The bizarre shooting was also witnessed by labourer James Cooper who saw Clasby walking away from Davis. "I did not hear the prisoner call out. If he had done so, I would have heard it. I told Davis 'You have shot Mr Clasby' but he said he did not. He did it himself. I told him I had seen him do it and Davis replied he did not care for he was on his post."

The convicts, he added, were about 50-60 yards away when the gun went off.

Haslar surgeon John Richardson confirmed the victim had died instantly from a gun shot wound to the head. "From what I saw of the prisoner during his examination before the magistrates, I am satisfied he is of unsound mind."

Beasley was then recalled to the witness box where Sewell questioned him about Davis's conduct. "He had been appointed pioneer a great part of his time because of his slovenly habits but I did not find anything that indicated insanity."

The prosecution's case at Hampshire Assizes the following March concluded with evidence from Mr Jacques who, as clerk to Fareham magistrates, had recorded his testimony after he had been cautioned. "He made a statement which I took down and now put in. It was a tissue of the most incoherent nonsense possible. I do not think he was capable of knowing what he was saying."

Davis denied the old fashioned worded charge of killing and slaying Clasby with a gun loaded with powder and ball and in his closing speech, Sewell urged jurors to accept the testimony of Richardson and Jacques that Davis was mentally ill.

But in his summing up, the judge, Mr Sergeant Atcherley, said he did not agree, telling jurors: "If any doubts arise in your mind, you must give the prisoner the advantage of it but I must candidly admit I can see none of it myself. Do you think the prisoner was in a state of responsibility at the time he did it?"

How long they spent in retirement is unknown but they ultimately returned a verdict of guilty and the judge sentenced him to 10 years transportation.

The 74-gun warship York was launched in Rotherhithe in 1807 but was converted into a floating prison in 1819, berthed in either Gosport or London, taking as many 500 convicts to the colonies. it was in regular service until 1848 when a serious rebellion broke out on board which led to its immediate withdrawl, and itwas broken up for scrap in 1858.