If we investigate, however, the connection between spots and terrestrial magnetic disturbances, we shall find altogether more satisfactory testimony. This evidence is of all degrees of strength, from probability up to what may be called certainty, and it is always obtained, not by a priori reasoning, but by the comparison of independent observations of something which has happened on the sun and on the earth. We will first take an instance of what we consider the weakest degree of evidence (weak, that is, when any such single case is considered), and we do so by simply quoting textually three records which were made at nearly the same time in different parts of the world in 1882.
A certain spot had been visible on the sun at intervals for some weeks; but when on the 16th of November a glimpse was caught of it after previous days of cloudy weather, the observer, it will be seen, is struck by the great activity going on in it, and, though familiar with such sights, describes this one as “magnificent.”
1. From the daily record at the Allegheny Observatory, November 16, 1882:—
“Very large spot on the sun; ... great variety of forms; inrush from S. E. to S. W.; tendency to cyclonic action at several points. The spot is apparently near its period of greatest activity. A magnificent sight.”
At the same time a sketch was commenced which was interrupted by the cloudy weather of this and following days. The outline of the main spot only is here given ([Fig. 49]). Its area, as measured at Allegheny, was 2,200,000,000 square miles; at Greenwich its area, inclusive of some outlying portions, was estimated on the same day to be 2,600,000,000 square miles. The earth is shown of its relative size upon it, to give a proper idea of the scale.
2. From the “New York Tribune” of November 18th (describing what took place in the night preceding the 17th):—
AN ELECTRIC STORM.
TELEGRAPH WIRES GREATLY AFFECTED.
THE DISTURBANCE WIDE-SPREAD.
... At the Mutual Union office the manager said, “Our wires are all running, but very slowly. There is often an intermission of from one to five minutes between the words of a sentence. The electric storm is general as far as our wires are concerned.”... The cable messages were also delayed, in some cases as much as an hour.
The telephone service was practically useless during the day.
Washington, Nov. 17.—A magnetic storm of more than usual intensity began here at an early hour this morning, and has continued with occasional interruptions during the day, seriously interfering with telegraphic communication.... As an experiment one of the wires of the Western Union Telegraph Company was worked between Washington and Baltimore this afternoon with the terrestrial current alone, the batteries having been entirely detached.