532 



Sir E. Sabine on Terrestrial Magnetism. 



[June 16, 



generating figure ; secondly, the discoidal coefficient m ; thirdly, the heli- 

 coidal coefficient n. Upon the relations of these parameters to each other 

 depends the shape of the shell. Thus in some n is nearly equal to m, and 

 in such cases the whorls scarcely embrace each other, and the figure pro- 

 duced is that of an elongated cone, as in the genera Turritella, Cerithium, 

 Acus, kc. Sometimes n exceeds m ; and in this case the resulting form is 

 an open spiral as in Vermetus, or a rapidly descending series of whorls. A 

 third possible case is that in which n is less than m, and the resulting 

 figure is globular ; but of this case, though a possible one, I hare not as 

 yet succeeded in obtaining an example. 



The following cases illustrate the formula n > m : — 





ft. 



m. 



Width c 



i who 





Amount of translation. 



Vermetns hunbricaHs . . 



: a 



1-3 



0-075 0-1 ! 



013 



| 0-173 



0*15 



0-22 



0 3 0-45 



Delphinula atrata 



600 



2* So 



0-013 »-5 



0-143 



041 



0 01 



0-05 



03 



The following instances exemplify the case n=m : — 



Species. 



n = m. 







Le 



ngth of whorls in decin 



ials c 



f an i 



rich. 



Helicostyla polychroa 



2 



0-41 



o-cs 



rise 



0-32 



0-7 















1-71 



0-09 



014 



0 26 



043 



076 

















PhasianeHa bulimoides . . 



1-8 



0 07 



0-12. 



0 23 



0-45 





















156 



005 



0-07S 013 



0-2 



0-32 



052 

















1-5 



015 



0'225 0 343 



054 



0-S4 



















1-434 



0-245 



04 



0-573 



0-32 



















Trochus niloticus 



1-41 



02 



03 



0 425 



0-63 



0-9 



1-2 















Fusus lonzissimus 



1-341 



0-25 



03 



044 



0-6 



031 



















1-33 



0-15 



02 



026 



0-35 



0-42 



054 



0S3 















1-33 



013 



0-17 



0-29 



0-3S 



051 

















Ac us dimiiLita 



1-277 



02 



0-267 0 31 



0-4 



052 



0-62 



0-88 















1-25 



0-15 



0-1 7t 



)0-23 



0-20 





045 



053 



07 



••9 











1-25 



02 



0"25 



0-32 



0-38 





H 















v ; ?~. " .v. u /-i r. - .vi- jsum . . 



1-24 



023 



03 



0-37 





:r 



















1-23 



0-03 



0-12 



015 



0178 





0-2S 



035 















1-22 



0-15 



01S2 022 



0-27 



034 



0 42 



05 















1-23 



0-07S 



01 



0-125 



0-16 



0-2 ! 



0-24 



03 



0"26 



044 



0-53 



0-625 



0-75 





1-163 



0-175 



0-3 



0-23 



0-265 



032 



0-367 



0-432 



047 



0-641 





Telescopium fuscum 



1-14 



01 



0-112 0125 



0-15 



018 



0-2 



0 24 



0-28 



0^25 



0^65 







VL ^Contributions to Terrestrial Magnetism. — Xo. XXL The 

 Magnetic Survey of the British Islands, reduced to the epoch 

 of 184:2-5." By General Sir Edward Sabine, K.C.B., Pre- 

 sident of the Royal Society. Received June 15, 1870. 

 (Abstract.) 



This paper contains a statement of the origin, progress, and completion of 

 this survey. It is accompanied by maps of the declination, inclination, 

 and magnetic force, which have been drawn at the Hydrographic Office of 

 the Admiralty under the superintendence of Captain Frederick John 

 Evans, R.N., F.R.S. The paper consists in great measure of Tables, giving 

 the observation of each of the three magnetic elements, with reductions in 

 every case for the secular change between the date of the observation and 

 that of the epoch (1842-5) for which the maps are constructed. 



