325 



the circle. The succeeding divisions, to the sixteenth, are all made 

 in the same mariner. In the next place, the error of the second bi- 

 sectional dot is to be set off by the micrometer head of the first mi- 

 croscope ; and the contemporaneous coincidence of this dot, with that 

 of the seventh of the succeeding small divisions of the sector, is to 

 be observed, and then the sector must be moved backwards upon its 

 axle sixteen divisions ; so that it will have to move forward again by 

 the motion of the circle one eighth of a division before the seventeenth 

 division upon the circle is to be cut. The succeeding divisions follow 

 in due course to the thirty-second, when allowance must be again 

 made for the known error of the third dot, and the work proceeds in 

 the same manner to the completion of the circle. 



In the application of this method to the instrument now construct- 

 ing for the Royal Observatory, which is to be divided on its edge, 

 instead of having the divisions upon the face of the instrument, nothing 

 new in principle is requisite, but merely a new position given to the 

 roller, and other apparatus employed ; but as that instrument may 

 deserve a particular description, the author hopes to have an oppor- 

 tunity of giving an account of its construction, to the Society, at no 

 very distant period. 



A Letter on a Canal in the Medulla Spinalis of some Quadrupeds. In 

 a Letter from Mr. William Sewell, to Everard Home, Esq. F.R.S. 

 Read December 8, 1808. [Phil. Trans. 1809, ;;. 146.] 



The canal, which is the subject of this letter, appears to have been 

 discovered by the author in the year 1803, although no account has 

 been given of it till the present description was drawn up at the re- 

 quest of Mr. Home. 



From the extremity of the sixth ventricle of the brain in the horse, 

 bullock, sheep, hog, and dog (which corresponds to the fourth ven- 

 tricle in the human subject), a canal passes in a direct course to the 

 centre of the spinal marrow, and may be discovered in its course by 

 a transverse section of the spinal marrow in any part of its length, 

 having a diameter sufficient to admit a large-sized pin ; and it is 

 proved to be a continued tube, from one extremity to the other, by 

 the passage of quicksilver in a small stream in either direction 

 through it. 



This canal is lined by a membrane resembling the tunica arach- 

 noidea ; and it is most easily distinguished where the large nerves 

 are given off in the bend of the neck, and at the sacrum. 



A numerical Table of elective Attractions ; with Remarks on the Se- 

 quences of double Decompositions. By Thomas Young, M.D. For. 

 Sec. R.S. Read February 9, 1809. [Phil. Trans. 1809, p. 148.] 



The attempts that have been made by some chemists to represent 

 the attractive forces of chemical bodies by number, having been li- 

 mited and hastily abandoned, *ome important consequences which 



