﻿390 Proceedvngs. 



liistoiy roughly to predict the futui'e cues. Owing to the complexity of the 

 moon's motions, approximate truth only was obtainable, even down to the 

 eighteenth century, and the compiitation was made by the wrcuitous method of 

 the nonagesimal. In our day, owing to the perfection of the tables published in 

 the " Nautical Almanac," the computation of eclipses or of occultations of 

 stai'S by the moon is rendered, very easy. 



These phenomena ai-e predicted in the "Nautical Almanac" generally from 

 the centre of the earth, and their visibility is stated within certain parallels of 

 latitude. But, from the effects of parallax, these times may vary by two hours 

 either way for a place on the earth's surface, or the occultation may not occur 

 at all in some positions within the limiting parallels. 



An independent prediction is therefore required for every place ; this may 

 be made approximately by a graphical projection of the figures of the bodies 

 in their true proportions and relative positions, which requires only a pair of 

 compasses and a scale of chords, such as is to be found on a foot rule, so as to 

 show the times within a very few minutes. If accuracy is required, the true 

 relative positions of the two bodies can then be taken out for the assumed 

 times, and the error to be applied to these assumptions ascertained by simple 

 computations in spherical and plane trigonometry. 



In our day the mere occurrence of the phenomena at the exact times pre- 

 dicted for them has lost the interest it had for our forefathers, to whom it 

 appeared wonderful and mysterious ; but a new source of interest in total 

 eclipses of the sun has arisen, from the means which the spectroscope and 

 polariscope afford for investigating the curious phenomena of the corona, red 

 flamies and sierra, and the means which photography supplies, with the aid of a 

 suitable telescope, equatorially mounted with clock motion to keep the object 

 steadily in the camera, of taking accurate pictures of the momentary appear- 

 ances. 



The various hypotheses to account for these appearances have now subsided 

 into a clear knowledge that the sun is surrounded to a depth of two or three 

 seconds of arc, corresponding to a depth of 900 to 1,200 miles,' by an atmos- 

 phere chiefly of hydrogen, but mixed with the incandescent vapours of many 

 other metals in a most tumtiltuous condition ; that prominences, consisting 

 essentially of hydrogen heated far beyond any temperature obtainable on this 

 earth, shoot out from it to a height of 90,000 or 100,000 miles in very short 

 intervals of time, and that outside that to an indefinite distance extends a 

 radiated luminous envelope, which cannot be considered material, but which 

 gives spectra strikingly similar to those of the Aurora and of the zodiacal light ; 

 that these effects seem to be associated with electrical discharges, and may 

 hereafter prove a visible link between radiated heat, light, and electricity, the 

 intimate connection of which is shown in many ways. 



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