Magnetic “Lines of Force”

@article{ERMagneticO,
  title={Magnetic “Lines of Force”},
  author={P. E.R.},
  journal={Nature},
  volume={60},
  pages={412}
}
IN some text-books and by some lecturers (e.g. Prof. A. Gray, as reported in NATURE of August 17, p. 379), the lines of magnetic force are said to be the curves along which iron filings are marshalled when sifted over a piece of card laid over a horizontally placed magnet.