@inproceedings{e.1883sur,
title={Sur les couches de niveau},
author={E. Beltrami},
url={http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108202931091609756},
booktitle={Acta Mathematica},
volume={3},
number={1},
pages={141-152},
year={1883},
}