342 



which it is attached, and which necessarily limits the precision of 

 which it is capable. 



The author next gives the results of some experiments with a col- 

 limator made for Captain Foster, having a float of only 5 inches in 

 diameter, and with a telescope 5 inches long; the errors generally do 

 not amount to more than -t^ths of a second. 



He then enters into details as to the manner of using the vertical 

 floating collimator in astronomical observations, beginning with the 

 portable azimuth and altitude circle, described by the Rev. F. Wol- 

 laston in his Fasciculus Astronomicus, and applicable to other similar 

 instruments. The new collimator affords also the most perfect me- 

 thod of adjusting the line of collimation of a mural circle or of placing 

 it at right angles to the axis. 



The author next proceeds to describe the method of applying the 

 instrument to the zenith telescope. In comparing the observations 

 made by the zenith sector, belonging to the Board of Ordnance, with 

 the zenith telescope used in conjunction with the vertical floating col- 

 limator, the mean of errors in the former case was + 0"'54 and 0"'75; 

 in the latter+ 0''"44 and 0"'66. From observations made on y Dra- 

 conis, the zenith distance of which at Greenwich is 2' 6"'36, and 

 at York Gate 0' 35"' 67 ; the difference of latitude between the two 

 places was found to be 2' 42"'03 ; that of Greenwich being 51 

 28' 38"'96, and of York Gate 51 31' 20"'99. The decimals of a se- 

 cond, by the azimuth and altitude circle and the horizontal floating 

 collimator, were '94; by the same instrument and the vertical floating 

 collimator, '76 ; and by the zenith telescope, and the vertical floating 

 collimator, '92 : the mean being *9. 



From the greater degree of precision attainable by the employment 

 of the vertical floating collimator, from the facility of its construction, 

 the readiness of its application, and the time saved by using it, the 

 author deems it not unreasonable to infer, that ere long, the use 

 of the level and plumb-line in celestial observations will be wholly 

 abandoned. 



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 17, 1828. [Phil. Trans. 1828,^. 291.] 



The author observes that opinions differ as to the elevation of the 

 Aurora Borealis above the surface of the earth, and that this is a 

 point which can be determined only by a series of concurring ob- 

 servations. The appearance of a phenomenon of this kind on the 

 29th of March, 1826, assuming the form of a regular arch at right 

 angles to the magnetic meridian, and marked by peculiar features, 

 continuing for above an hour in the same position, afforded a most 

 favourable opportunity for obtaining the data requisite for the solu- 

 tion of this problem ; and the author accordingly took great pains 

 to collect as many authentic accounts as possible of the apparent 

 position of this luminous arch with reference to the stars, when seen 



