536 THE SCIENTIFIC MONTHLY
tail of the following night is very much broken, there are several fairly well-defined nuclei, and it is brighter than the tail of the twelfth. Two photographs of this comet were fortunately made on the second night, with a time interval of three quarters of an hour. A comparison of the positions of the three nuclei on the two plates showed that they had moved outward from the head with great speed during the interval. The nucleus nearest the head had traveled out with a speed of forty- four miles per second, the next nucleus with a speed of fifty-two miles a second, and the one still farther out with a speed of fifty-nine miles per second. Here are two photographs of comet Brooks (Fig. 9) made on October 21 and October 22, 1893, by Barnard. The structure of the tail on the first photograph is not at all the structure on the second. The tail of the first night has been scattered to invisibility and an abso- lutely new tail has replaced it. The outward motion of well-defined tail structure has been measured for many comets. Here is a series of measures made by Curtis upon points in the tails of Halley's comet.
AvEEAGE Velocities op Recession, from the head, of Matter in the Tail op
Halley's Comet
Date. 1910 Me&n Dtotonoe from Head Average Velocity
��May 23 800 m
May 27-28 400,000 m
May 25-26 930,000 m
June 2-3 1,360,000 m
May 28-29 1,730,000 m
June 6 2,200,000 m
May 26-27 2,500,000 m
May 30-31 6,600,000 m
June 7-8 8,400,000 m
��les 0.6 miles per sec.
les 8 miles per sec.
les 12 miles per sec.
les 20 miles per sec.
les 23 miles per sec.
les 27 miles per sec.
les 24 miles per sec.
les 45 miles per sec.
les 57 miles per sec.
��The points to be measured were not well defined, and the measures could not be accurate, but it is clear that high speeds and accelerated speeds prevailed. The tail materials start out slowly from the head, and increase their speeds with the distance from the head, as we should expect of motion resulting from the action of a continuous force which meets with no sensible resistance.
In Fig. 10 are reproductions of photographs of Halley's Comet made by Curtis on June 6 and June 7, 1910. A semi-detached part of the tail, seen on the photograph of June 6 about an inch above the head, is visible about two and a half inches above the head on the photograph of June 7. This structure was first observed by Curtis shortly after it had emerged from the central part of the head on June 4, and it was recorded on the photographs secured by a great many observatories in the following four days, as the rotation of the earth carried the comet successively into position for observation at the observatories. The times when the lower point of the structure had certain positions is
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