86 



Prof. W. C. Williamson on the Fossil [Dec. 19, 



these in steps exhibited in several successive Tables, results are found 

 for the magnetic disturbance in the three directions, longitudinal, trans- 

 versal, and vertical ; and these are compared with the terrestrial forces in 

 the same directions. The principal results are, that the transversal and 

 vertical parts of terrestrial magnetism are absolutely neutralized, and the 

 terrestrial longitudinal force is diminished by about one fourth part. 



An anomaly presented itself in the results for the station at the middle 

 of the Anglesey Wate-tube, which induced the author to inquire of 

 Edwin Clark, Esq. (the actual Superintendent of the construction of the 

 bridges), whether any peculiarity of material could explain it. Mr. Clark 

 immediately remarked that this was the tube which, in the act of raising 

 the sections of the tube, sustained a fall of several inches, undoubtedly 

 straining the tube very greatly. It appears almost beyond doubt that this 

 accident is the cause of the anomalous magnetic condition of that section 

 of the tube. Mr. Clark remarks that it is an interesting fact, if esta- 

 blished, that the result of an accident which occurred about a quarter of a 

 century ago can still be traced in the magnetic condition of the tube. 



The results in the Conway bridge, for transversal and vertical action, 

 are in perfect agreement with those for the Britannia bridge ; but the 

 results for longitudinal disturbance do not present the same agreement. 

 The Conway bridge, however, is nearly at right angles to the magnetic 

 meridian ; and it appears probable that its magnetical state depends on 

 the circumstances of its construction, when its Holyhead end was in a di- 

 rection about 48° 50' west of north, in which position it would receive 

 much magnetism, which the forces subsequently acting on it could not 

 remove. 



II. " On the Organization of the Fossil Plants of the Coal-mea- 

 sures. — Part IV. Dictyoxylon, Lyginodendron, and Heteran- 

 gium." By W. C. Williamson, F.R.S., Professor of Natural 

 History in Owens College,Manchester. Beceived December 4, 

 1872. 



(Abstract.) 



In 1866 Mr. Binney gave the name of Dadoxylon Oldhamium to a fossil 

 stem of a plant from the Lower Coal-measures of Lancashire, believing 

 it to belong to the same class of G-ymnospermous Exogens as the Pinites 

 of Witham and the Dadoxylon of Endlicher. In 1869 the author pointed 

 out that the reticulated markings upon the surface of its vessels were 

 modifications of the spiral fibre of fibro-vascular tissue, and not the disks 

 of what is often designated glandular fibre. He consequently separated 

 the plant from the Dadoxylons under the name of Dictyoxylon Oldhamium. 

 At the Edinburgh Meeting of the British Association in 1871 he gave a 

 brief account of the structure of this plant, as also of what appeared to be 

 a second species from the Lower Coal-measures of Burntisland in Eife- 



