1868.] Mr. C. Maxwell^, Comparison of Electrostatic with ^c. 449 



boues of the face, and that of the inferior maxilla is specially referred to. 

 This bone is beginning to ossify in a foetus nine-tenths of an inch long, and 

 is distinctly formed in a foetus one inch and one-tenth long. Each half 

 may be said to grow from four centres, formed (1) by the cartilage which 

 tips the condyloid extremity, (2) by the layer of membrane in front of 

 jMeckel's cartilage, (3) by the ossification of the anterior extremity of 

 Meckel's cartilage, (4) by deposits of bone in the perichondrium of the 

 anterior and middle thirds of the same cartilage, from which is derived the 

 plate of bone which forms the base of the dental canal. 



After giving the measurements of the angles formed between the ascend- 

 ing ramus and the body of the bone, and after referring to the subdivisions 

 of the groove for the teeth, the growth of the mylo-hyoidean ridge is de- 

 scribed, as well as the ossification of the anterior extremity of Meckel's 

 cartilage, the latter forming a triangular block beneath the incisor sockets, 

 to the twist acquired by which the prominence of the front of the maxilla, 

 known as the mentum or chin, appears to be due. In a foetus four inches 

 and seven-tenths long, the block of bone formed in the anterior extremity 

 of the cartilage of Meckel is still clearly defined. 



VIII. ^' On a Method of making a Direct Comparison of Electrostatic 

 with Electromagnetic Force ; with a Note on the Electromag- 

 netic Theory of Light.^' By J. Clerk Maxw^ell, F.H.SS.L. & E. 

 Received June 10^ 1868. 



(Abstract.) 



The experiments described in this paper were made in the laboratory of Mr. 

 Gassiot, who placed his great battery of 2600 cells of bichloride of mercury 

 at the disposal of the author. Mr."Willoughby Smith lent his resistance-coils 

 of 1,102,000 Ohms; Messrs. Forde and Fleeming Jenkin lent a sensitive 

 galvanometer, a set of resistance-coils, a bridge, and a key for double 

 simultaneous contacts ; and Mr. C. Hockin undertook the observation of 

 the galvanometer, the adjustment of the resistances, and the testing of 

 the galvanometer, the resistance-coils, and the micrometer-screw. The 

 electrical balance itself was made by Mr. Becker. 



The experiments consisted in observing the equilibrium of two forces, 

 one of which was the attraction between two disks, kept at a certain differ- 

 ence of potential, and the other was the repulsion between two circular 

 coils, through which a certain current passed in opposite directions. For 

 this purpose one of the disks, with one of the coils attached to its hinder 

 surface, was suspended on one arm of a torsion-balance, while the other 

 disk, with the other coil behind it, was placed at a certain distance, which 

 was measured by a micrometer-screw. The suspended disk, which was 

 smaller than the fixed disk, was adjusted so that in its position of equi- 

 librium its surface was in the same plane with that of a " guard-ring," as 



