Meteorological observations and essays / By John Dalton.

  • John Dalton
Date:
1834
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Licence: Public Domain Mark

Credit: Meteorological observations and essays / By John Dalton. Source: Wellcome Collection.

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    observations were made: if it seemingly touched the land, then its distance from the observer might be either one hundred yards or one thousand miles ; if it sunk 1°, 2° or 3° below the summit of the land, then its distance could not exceed two thousand two hundred and fifty, one thousand five hundred, or seven hundred and fifty yards respectively, and might have been half of those distances or less. It is to be regretted that this so favourable an oppor- tunity should have been let slip for want of more precise observation : how far did the beam descend below the mountain? how long did it continue there? did it ascend again, or did it die away there? These are inquiries which every one would wish to make upon such an extraordinary event, but we find no answer. It is evident then that nothing certain, nor even probable, can be obtained as to the real height of this beam; the observation may just as readily be adapted to one extreme of height as to the other, or to any intermediate one. On the whole, I have very little addition to make to the essay on the aurora borealis, as far as regards its theory given in 1793, in the fourth sec- tion, page 167. The aurora consists of luminous arches or rings, drawn round the magnetic poles, in the manner of parallels of latitude around the poles of the earth on a terrestrial globe, and of lu-