We have already stated that the potential necessary to produce a discharge in partially exhausted tubes diminishes with the pressure until a certain pressure of minimum resistance has been reached, but as the rarefaction is increased beyond this point, then the potential has to be increased in order to produce a discharge. The experiments on which this result was founded were made with a tube 33 inches (83∙8 centims.) long and 2 inches (5.1 centims.) in diameter, and it was found that in a hydrogen partial vacuum the pressure of minimum resistance was 0∙642 m.m., 845 M. Professor Stokes suggested the desirability of making experiments with a wide tube, or, still better, with a globe, as it would allow the discharge to spread laterally and diminish the resistance thereby, and, very probably, alter the pressure of minimum resistance. He considers this to be a question of importance with reference to the height of the aurora, for in the atmosphere there is ample space for lateral expansion, and it is conceivable that the least resistance may correspond to a pressure a good deal different from that which gives the least resistance in a tube. Experiments were made in an air residue contained in a vessel in some measure resembling a prolate spheroid, 7 inches (17∙8 centims.) long and 5 inches (12∙7 centims.) in diameter, the distance between the terminals, one a cup, positive, the other heart-shaped, negative, was 3 5/8 inches (9∙2 centims.) (see fig. 1). The battery employed consisted of 11,000 cells, giving a deflection when short-circuited of 49° with a total internal resistance of 360,800 ohms.