The swell-mob--the London branch of which is said to consist of from one hundred and fifty to two hundred members--demand the greatest amount of vigilance to detect. They hold the first place in the "profession." Their cleverness consists in evading the law; the most expert are seldom taken. One "swell," named Mo. Clark, had an iniquitous career of a quarter of a century, and never was captured during that time. He died a "prosperous gentleman" at Boulogne, whither he had retired to live on his "savings," which he had invested in house property. An old hand named White lived unharmed to the age of eighty; but he had not been prudent, and existed on the contributions of the "mob," till his old acquaintances were taken away, either by transportation or death, and the new race did not recognize his claims to their bounty. Hence he died in a workhouse. The average run of liberty which one of this class counts upon is four years. The gains of some of the swell mob are great. They can always command capital to execute any especial scheme. Their traveling expenses are large; for their harvests are great public occasions, whether in town or country. As an example of their profits, the exploits of four of them at the Liverpool Cattle Show some seven years ago, may be mentioned. The London Detective Police did not attend, but one of them waylaid the rogues at the Euston Station. After an attendance of four days, the gentleman he was looking for appeared, handsomely attired, the occupants of first-class carriages. The Detective, in the quietest manner possible, stopped their luggage; they entreated him to treat them like "gentlemen." He did so, and took them into a private room, where, they were so good as to offer him fifty pounds to let them go. He declined, and over-hauled their booty; it consisted of several gold pins, watches, (some of great value,) chains and rings, silver snuff-boxes, and bank-notes of the value of one hundred pounds! Eventually, however, as owners could not be found for some of the property, and some others would not prosecute, they escaped with a light punishment. In order to counteract the plans of the swell mob, two of the sergeants of the Detective Police make it their business to know every one of them personally. The consequence is, that the appearance of either of these officers upon any scene of operations is a bar to anything or anybody being "done". This is an excellent characteristic of the Detectives, for they thus become as well a Preventive Police. We will give an illustration:--