"We have no means of surmising the method by which this man succeeds in arresting vibratory motions of certain wave-lengths," said Eldridge didactically, "any more than we are able to define the precise nature of electricity. But, as in the case of electricity, we can observe the action of its phenomena. Two salient features leap out at us: one is that these phenomena are limited in time; the other that they are limited in space. The latter aspect we will examine, if you please, gentlemen. "The phenomena have been directed with great accuracy (a) at the Atlas Building; (b) at this city and some of its immediate suburbs. The peculiarity of this can not but strike an observant mind. How is this man able, at forty or fifty miles distance, to concentrate his efforts on one comparatively small objective? We can only surmise some system of insulating screens or focal mirrors. I might remark in passing that the existence of this power to direct or focus the more rapid ethereal vibrations would be a discovery of considerable scientific moment. But if this is the method employed, why do we not cut a band of vibratory nullifications, rather than touch upon a focal point?" "Repeat softly," murmured the irrepressible _Register_ man. "Why," explained Eldridge patiently, "are not the people and buildings between here and the unknown operator affected? The only hypothesis we are justified in working upon is that the man's apparatus is at a height sufficient to carry over intervening obstacles. This hypothesis is strengthened by the collateral fact that the territory we have just determined as that within which he must be found lies in the highlands of our own and neighboring states. We may, therefore, eliminate the low-lying districts within our radius." Percy Darrow opened one eye. "Perhaps he's up in a balloon," he drawled languidly; "better take along an aeroplane." Eldridge cast on him a look of cold scorn. Darrow closed the eye. CHAPTER XVII DRAWING THE NET The "zone of danger", as the _Bulletin_ named it, was immediately the scene of swarming activities. Besides the expedition immediately despatched by the interests backing the investigation, several enterprising newspapers saw a fine chance for a big scoop, and sent out much-heralded parties of their own. The activities of these were well reported, you may be sure. Public interest was at once focused reassuringly on the chances of finding the annoying malefactor to-day or to-morrow; there no longer existed a doubt that he would be found. The weight of dread was lifted, and in the reaction people made light of the inconveniences and fun of the menacing messages that now came in by the dozen. [Illustration: "Let Eldridge do his best."]