3. The second region or stratum of air terminates I suppose where the twilight ceases to be refracted, that is, where the air is 3000 times rarer than at the surface of the earth; and where it seems probable that the common air ends, and is surrounded by an atmosphere of inflammable gas tenfold rarer than itself. In this region I believe fire-balls sometimes to pass, and at other times the northern lights to exist. One of these fire-balls or draco volans, was observed by Dr. Pringle and many others on Nov. 26, 1758, which was afterwards estimated to have been a mile and a half in circumference, to have been about one hundred miles high, and to have moved towards the north with a velocity of near thirty miles in a second of time. This meteor had a real tail many miles long, which threw off sparks in its course, and the whole exploded with a sound like distant thunder. Philos. Trans. Vol. LI. Dr. Blagden has related the history of another large meteor, or fire- ball, which was seen the 18th of August, 1783, with many ingenious observations and conjectures. This was estimated to be between 60 and 70 miles high, and to travel 1000 miles at the rate of about twenty miles in a second. This fire-ball had likewise a real train of light left behind it in its passage, which varied in colour; and in some part of its course gave off sparks or explosions where it had been brightest; and a dusky red streak remained visible perhaps a minute. Philos. Trans. Vol. LXXIV. These fire-balls differ from lightning, and from shooting stars in many remarkable circumstances; as their very great bulk, being a mile and a half in diameter; their travelling 1000 miles nearly horizontally; their throwing off sparks in their passage; and changing colours from bright blue to dusky red; and leaving a train of fire behind them, continuing about a minute. They differ from the northern lights in not being diffused, but passing from one point of the heavens to another in a defined line; and this in a region above the crepuscular atmosphere, where the air is 3000 tines rarer than at the surface of the earth. There has not yet been even a conjecture which can account for these appearances!--One I shall therefore hazard; which, if it does not inform, may amuse the reader.