For instance, suppose a stain on the clothes of a person accused of murder were examined by a chemist for the prosecution and found to consist of blood. The fact would tell against the accused, even though the witness (as in a recent case) could express no opinion whether it was human blood, or the blood of an animal. Assuming in this hypothetical trial that the blood stain was really due to rabbit's blood, another chemist representing the prisoner might be acquainted with the comparatively recent physiological methods of distinguishing between the blood of different animals, and thus be able to prove the real nature of the blood stain and break one of the links in the chain of evidence. In most of the important criminal trials the scientific evidence is given by more than one witness, and the possibility of mistake is thus greatly reduced, but this is not invariably the rule. Scientific criticism derived from a first-hand examination of the material would be of much more value than the criticism of the statement of the results, and might have considerable weight upon the conclusions of the jury. A defending counsel cross-examining a scientific witness is usually dealing with an unfamiliar subject, and lacks the specialised knowledge that would enable him to point out the weak points in the evidence. When a wealthy person is on trial, however, the counsel has the advantage of getting the best expert advice upon the scientific matters put forward in evidence, and is thus able to lay stress on all that will help his client, but a poor prisoner lacks this advantage, and therefore runs a greater chance of being convicted. An early trial in which the prisoner owed his acquittal to a conflict of scientific evidence was that of Spencer Cowper, the grandfather of Cowper, the poet, who was tried at the Hertford Assizes in 1699 for the murder of a young gentlewoman named Sarah Stout. With Cowper were also tried several of his friends, whose remarks having been overheard had suggested that they were aware of what had happened to the girl. Cowper, who was a barrister, defended himself and incidentally his companions. The story told by the prosecution was that at the previous Assizes the prisoner had stayed for a night at the house of Mrs. Stout, the mother of Sarah Stout. The servant-maid stated that she had been told to prepare Mr. Cowper's bed, and that when she came downstairs again, it then being about eleven o'clock in the evening, he had gone out, presumably with Sarah Stout, who was never again seen alive. The next day her dead body was found floating upon the river.