XXIV
HALLEY'S COMET AND ELECTRICAL WAVES
Fig. 196
It was the year 1910 and Halley's comet was approaching the sun. On May 18 its tail might be expected to reach the earth. Astronomers had requested all who might be possessed of wireless telegraph apparatus to watch on that day for any peculiar behaviour of their apparatus so that evidence might be obtained whether or not the comet sends forth such ether waves as we call electricity. Harold desired me to explain the whole matter to his group of friends, which I did on a subsequent evening, as follows:
"Although Halley's comet has come within the earth's orbit about three thousand times since its first recorded appearance, I know of no man living who can give a satisfactory account of having seen it. Any one who has seen it before must be at least seventy-five years old, for it requires seventy-five years to make one complete circuit of its own orbit. But no one who is now seventy-five could have observed it intelligently, and even one who is now eighty-five years old would have to tell what he saw when he was ten years old and has remembered for seventy-five years. Furthermore, any account of how it looked on a former return is no guide to how it may appear on this trip. You may properly think of the comet as a group of solid pieces no bigger than the stones you may throw, scattered, two or three to the mile, through a space 12,500 miles broad. This extremely thin cloud of particles does not reflect enough sunlight to be visible, even in a telescope, in any part of its journey, and hence we should be wholly unaware of its existence if it did not sometimes have the strange faculty of giving out light of its own while in that part of its own orbit nearest to the sun. At such a time there is a hazy light enveloping the mass of small bodies, and streaming away sometimes many million miles from them. The mass of small bodies is generally referred to as the nucleus, and the stream of luminous gas which the nucleus gives forth is called the tail, though it reminds me more of a search-light.
"It does not trail along behind the comet but always points away from the sun ([Fig. 197]). The normal thing for a comet to do is to begin to develop a faint light and a short streamer as it gets near to the sun, to have its light grow brighter and its streamer to grow longer until it reaches the point nearest the sun, and then to have its light grow dimmer and the streamer grow shorter as it recedes from the sun.
