Apparent and real dimensions of the largest tails on record–Formation and development of cometary appendages; their disappearance–Variations of length in the tail of Halley's comet at its different apparitions–Great comet of 1858, or comet of Donati.

Since we have entered upon the statistics of various cometary elements, let us here give a few particulars respecting the real and apparent dimensions of cometary tails. We will first confine ourselves to the maximum dimensions under which they have been viewed from the earth, dimensions measured in degrees, according to the apparent extent occupied by the train itself in the celestial vault. Passing, then, from the apparent lengths, we will proceed to the actual measures expressed in miles. Under the first head the scale of magnitude will be found to include an enormous range, varying from the tail of 2½°, belonging to the comet of 1851, to the immense tail of 100°, possessed by the comet of 1264, and to the still greater tail of the comet of 1861, which attained a length of 118°, thus exceeding by 28° the apparent distance between the horizon and the zenith. Nor are the differences less considerable when we compare the true dimensions.