"Again, there is Fort Yuma in Arizona, the hottest place in the United States, with the possible exception of Death Valley. Yes, it is abnormally hot at Yuma and the air is furnace-dried; the old-timers will tell you that, on really bad days, a man can't drink water fast enough to keep from dying of thirst. Of course, men do die from the effects of the heat, but it isn't our ordinary form of sunstroke. To sum up, then: "No sunstroke at New Orleans, where it is abnormally humid and hot; and none at Fort Yuma, where it is abnormally dry and hot. But plenty of cases in Paris, Chicago, and New York, where the climate is supposed to be temperate. "The inference is logical: under certain conditions, one of the invisible, high frequency rays, always present in sunlight, is enabled to get in its deadly work. Unfortunately, we don't know what those conditions are. Perhaps the proportion of static electricity in the atmosphere may have something to do with it. Anyway, the fact remains that men do die of heat stroke in New York and Paris, while Louisiana and Florida are comparatively free from that particular peril to life." "Then, according to your theory, it is the Sigma ray which is the active lethal agent in sunlight?" "Yes, and Fielding Thaneford's invention enabled him to isolate the ray in question, at the same time enormously intensifying its action. Both Graeme and John Thaneford died the instant that it touched them." "And that was Fielding Thaneford's secret," said Betty, just returned from a flying visit to the nursery, where Little Hugh lay sleeping. "Such a horrible secret!" She shuddered. "Just as well that it died with him," assented Warriner soberly. "Still, in the end, he sought to stop the evil thing that he had set in motion," persisted Betty. "He told me all he could; all indeed that it was necessary to know, once I really began to use my wits." "Which reminds me," I put in, "that you have yet to explain how you finally managed to read the cypher. What put you back on the track?" "So simple a thing it was, too," laughed Betty. "And so easy to overlook." "I remember years ago," remarked Warriner, "that, on account of certain rare astronomical conditions, it was possible to see the planet Venus at midday. It took me the longest time to find the star, although I thought I knew just where to look; also all my friends were admiring the spectacle. At last I saw it, and then it was an easy matter to locate it again. I suppose the reason is that I didn't know what to expect; some sort of junior sun, I reckon. In reality, it was only a pin-point of light, but brilliant as a diamond." "And there's the game of challenging an opponent to find a word in a geographical map," said Betty. "It isn't the one printed in fine type and tucked away in a corner that is so hard to discover. The really invisible word is the one stretching in big, widely separated letters clear across the page." "Will you _tell_ me?" I asked impatiently.