Page:The story of the comets.djvu/185

X. The following is a table of the dimensions of the comet's nucleus and tail, at the undermentioned dates :—

The head of Donati's Comet deserves some special description because of the changes which it underwent and which have already been mentioned as features which often characterise very large comets. Bond first noticed, on Sept. 20, envelopes, 2 in number, above the nucleus, the outer one at a distance of 16" above the nucleus, and the inner one about 3". The outer one disappeared on Sept. 30 at the height of about 1'. Meanwhile a third had appeared, the one originally second having gradually expanded so as to take the place of the first. Seven successive envelopes in all were seen to rise from the comet, the last one starting on Oct. 20, when all the others had been dissipated. It was calculated that the