In N.L.R.B. v. Sands Mfg. Co., 306 U.S. 332, 59 S.Ct. 508, 83 L.Ed. 682, affirming the ruling of this Court in 6 Cir., 96 F.2d 721, the Supreme Court held the employer justified in abandoning further negotiations with the Union when conditions showed the uselessness of continuing with them.