Mr. Gray's Description of a new Kind of Pear-Encrinite. 219 



cle in one direction, by the addition of a telescope fixed on 

 a support at such a distance from the circle that the collimator 

 maybe conveniently placed between, and in a line with, the two 

 telescopes. To find the error of collimation with the telescope 

 of the circle directed towards the north, proceed as follows : 



1. The line of collimation of the telescope of the collimator 

 being very nearly horizontal, place the instrument, with its 

 telescope looking to the south, to the north of the mural circle. 

 2. Make the line of collimation of the telescope of the circle, 

 (pointing northwards,) parallel to that of the telescope of the 

 collimator, and read off the (minute) angle of elevation or 

 depression. 3.. Turn the collimator half reund in azimuth, 

 when its telescope (pointing northwards) should be in a line 

 with the fixed telescope placed to the north of the collimator. 

 Make the line of collimation of the fixed telescope, pointing 

 southwards, parallel to that of the telescope of the collimator. 

 4. Remove the collimator, and measure by the micrometer of 

 the fixed telescope the vertical angle formed by the intersec- 

 tion of its line of collimation by that of the telescope of the 

 circle ; half of which angle is the correct horizontal inclina- 

 tion of the line of collimation of the telescope of the circle*. 



Having thus determined the northern error of collimation, 

 we may subsequently ascertain, after the same method, the 

 southern one ; and on comparing these errors with that given 

 by the vertical collimator with the telescope pointed towards 

 the zenith, we obtain the horizontal flexure of the telescope 

 for both directions. I am, Gentlemen, yours, &c. 



Leeds, Aug. 2, 1828. John Nixon. 



XXXVIII. Description of a newKind o/ % Pear-Encrinite i /c>ttwe? 

 in England. By John Edw. Gray, Esq. F.G.S.Sfc.f 



Encrinites (Apiocri?iites) Prattii, n. 



Specific Character : — /COLUMN formed of round joints ad- 

 ^^ hering by radiating surfaces.? of 

 which the 4? or 5 top ones gradually enlarge at the apex, and 

 sustain the pelvis, &c. 



Icon. n. Inhab. Lias, summit of Lansdown, near Bath. 

 J. S. Pratt, Esq. Mus. Brit. 

 This species appears to be intermediate between A. rotundus 

 and A. cllipticus of Miller, and for the sake of comparison 

 I have given the specific character after his method. 



* This angle will be an elevation, or a depression, according as the line 

 of collimation of the fixed telescope points above, or below that of the 

 telescope of the circle. f Communicated by the Author. 



2 F 2 The 



