The comparison then between the zenith sector and the zenith telescope used with the vertical floating collimator, will stand thus: I shall now proceed to deduce the latitude of York Gate from the observations made with the zenith telescope. The polar distance of y Draconis for January 1827, Difference of latitude between Greenwich and York Gate Latitude of Greenwich Latitude of York Gate 0° 2' 6".36 0 0 35.67 0 2 42.03 * It may be remarked, that if the first eight observations and the last seven had been taken to form the two sets, the result would have been less favourable to the zenith telescope. But in the last seven observations there would then have been only a single observation less than the mean; and this, as I have before said, is an inadmissible case for comparison. Were this observation excluded, and the two sets made to consist of eight and of six observations, the resulting differences from the mean would have been far more favourable. The following is a view of the results, As any difference in the tables of refraction employed will equally affect the latitude and the zenith distance deduced from it, no correction is necessary on this account. We have then for the latitude of York Gate, By the azimuth and altitude circle and the hori-351° 31′ 20′′.94 zontal floating collimator. By the same instrument and the vertical floating col- 51 31 20.76 limator. By the zenith telescope and the vertical floating col- 51 31 20 .99 limator In my description of the horizontal floating collimator, I have recommended it to be employed in an observatory as a fixed point; its zenith distance being determined by means of the vertical floating collimator. For this purpose the box should be of cast iron, the openings in the ends of the box closed by pieces of plane glass, and the cover rendered air-tight. We have seen that the error in the vertical floating collimator is scarcely appreciable, though the mercury and float are agitated by turning the instrument half round; and it is not too much to anticipate, that where there is no such cause of disturbance, the horizontal floating collimator will suffer no change of inclination. This, however, may readily be ascertained by experiment. If I have succeeded in the object of this paper, I shall have demonstrated that the vertical floating collimator is an instrument capable of determining the zenith point with a precision hitherto unknown; that by its aid a meridional observation of an altitude or of a zenith distance may be completed, not only the same evening, but within the space of a very few minutes, and that too, without the necessity of turning the circle in azimuth. These are advantages which no other method of observing affords, and which astronomers well know how to appreciate. If to these be added the facility with which the floating collimator may be constructed, the ease with which it is used, and its general applicability to all astronomical circles, whether small or of large dimensions, it may not perhaps be too much to infer that ere long the use of the level and of the plumb-line in celestial observations will be wholly abandoned. XIII. On the height of the Aurora borealis above the surface of the earth; particularly one seen on the 29th of March, 1826. By JOHN DALTON, F.R.S. Read April 17th, 1828. APPREHENDING that the Royal Society will favourably receive accounts that have a direct tendency to determine the height of that interesting phænomenon, the Aurora borealis, I have been induced to transmit some observations that were made upon a very remarkable one, which appeared in the evening of the 29th of March, 1826. From some recent observations, an opinion seems to be entertained by some writers, that the aurora is not so high as has generally been estimated; but it is only from facts and observations such as the following, I conceive, that any near approximation to the true height can be obtained. The aurora borealis above mentioned, was of a kind very rarely occurring. It assumed the appearance of a rainbow-like arch, stretching across the midheaven, at right angles to the magnetic meridian. It was subject to very little change of position for an hour or more, and therefore afforded time to observe the angle of its elevation above the horizon. In the period of five years' observations at Kendal formerly, above one hundred appearances of the aurora occurred to me, and only one of the kind just described. I had not an opportunity of seeing the one which is the subject of this paper, but it was seen here (at Manchester) by a friend of mine about 9 o'clock on his returning home from a visit to me. He did not indeed observe the luminous arch, either from its having vanished, or from the obscurity of our atmosphere; but he remarked some beams or corruscations in the north-western hemisphere, of a low altitude; and not having seen an aurora for a long time, he induced the family at home to go out and catch a glimpse of the phænomenon, now much more rarely seen than formerly. A few days afterwards I accidentally noticed a paragraph in the Lancaster Gazette describing the luminous arch of the aurora, as well as the accompanying appearances; and as such a striking and unusual phænomenon could not fail to attract general attention, I examined the provincial newspapers and other periodicals of the time, and took occasion soon after to make inquiries personally, or by writing, of such individuals of judgement as had seen the phænomenon in various places near the line of the magnetic meridian. The result was, a collection of a more complete and extensive series of observations than was ever before made, in all probability, towards determining the height of the luminous arch of the aurora.-I shall now proceed to detail some of the particular observations. The accounts represent the arch to have been seen in places 170 miles distant in a north and south direction, and forty-five miles distant in an east and west direction, comprising an area of seven or eight thousand square miles; but it must have been much more extensively visible, as in most cases the writers of the different accounts describe their situation as central with regard to the phænomenon. It was seen at Edinburgh and Leith, Kelso, Jedbergh, and Hawick in Scotland; at Carlisle, Penrith, Keswick, Cockermouth, and Whitehaven in Cumberland; at Kendal and at Kirkby-Stephen in Westmorland; at Lancaster, Preston, Warrington, and Manchester in Lancashire; and at Doncaster in Yorkshire. Descriptions of the phænomena as seen at most of these places were immediately given in the newspapers of Lancaster, Kendal, Carlisle, Whitehaven, Kelso, &c., and some of these accounts were copied into the London papers soon after. All the accounts that I have seen from places between Lancaster and Edinburgh, as well as at these two places, agree that a luminous arch was first seen about 8 o'clock in the evening; that it continued without much motion for an hour nearly, and then gradually vanished, leaving the northern sky illuminated as usual after an aurora borealis of the common kind: so that it seems impossible to doubt that the same arch was seen at all the places of observation, and at the same time. A good description of the phænomenon was published by Messrs. Coldstream and Foggo in the Edinburgh Journal of Science for June 1826: it is as follows: |