FORCES ACTING IN MAGNETIC CIRCUITS. 197 
Plate XI. 
Fig. Fig. 
1. Semivertagus capillatus 7. Triploca ligata 
2. Acteopyramis olivelleformis 8. Basilissa Cossmanni 
3. Semivertagus subcalvatus 9. Litiopa punctulifera 
4. Risella alta 10. EHburnopsis tessellatus 
5. Harpa pachycheila 11. Colina fenestralis 
6. Ataxocerithium concatenatum 12. Crania quadrangularis 
Plate XII. 
1. Phragmorisma anatineformis 5. Myochama rugata 
2. Pholadomya australica 6. Poroleda lanceolata 
3. Anatina dolabreformis 7. Colina apicilirata 
4. Myochama plana 
Plate XITI. 
1. Conoclypeus rostratus 4, Laganum platymodes 
2. Astrangia tabulosa 5. Sismondia Murravica 
3 
. Arachnoides incisa 6. Strophodus eocenicus 
ON AN APPROXIMATE METHOD OF FINDING THE 
FORCES ACTING IN MAGNETIC CIRCUITS. 
By RicHarD THRELFALL, M.A., Professor of Physics, University 
of Sydney. 
Assisted by FLtorenceE Martin, Student in the University of 
7 Sydney. 
[With Plates XIV. and XV.] 
[Read before the Royal Society of N. 8. Wales, July 5, 1893. ] 
Durine the last three years I have had occasion to design a good 
many reciprocating electro-magnetic mechanisms, and have fre- 
quently felt the want of some simple method of making the neces- 
sary approximate calculations of magnetic forces. I have obtained 
very little satisfaction from the attempts I have made to calculate 
tractions proceeding by the method of finding poles and applying 
the law of inverse squares. This ill success led me to investigate 
the applicability of the methods established by Maxwell in the 
