A sharp distinction must be made between a trade union and an industrial union. One includes only workers in a particular trade. The other, and in this respect it resembles roughly the Knights of Labor, includes all workers of an industry such as mining or building. Such a union is the Industrial Workers of the World (I. W. W.), which contends that the interests of labor are too closely knit together to permit its members to be divided among numerous trade unions. Hence, the two types of unions are opposed to each other. The officers of the American Federation of Labor during the past few years have repeatedly criticized both the aims and the acts of the I. W. W., and warned members of trade unions not to affiliate themselves with that organization. The I. W. W. in its battles with employers has used a new weapon known as sabotage, which may be denned as a "strike on the job" - that is, a strike without quitting work. This is accomplished by disabling machinery or spoiling the product. In either case the employer soon knows that he is being fought by an unseen foe, for seldom is the guilty individual ever caught. Consequently, he must do one of two things: comply with the demands of his men or get a new labor force.