After much googling I finally found a copy of Robert MacDougall’s “The Structure of Simple Rhythm forms” from 1903. It was hidden in Harvard Psychological Studies, Volume 1 now freely available from Project Gutenberg ( https://www.gutenberg.org/ebooks/16266 . Since it seems the book is now in the public domain I took the liberty to convert “The Structure of Simple Rhythm forms” into pdf-format and post it here so that it might be more easily found in the future.

MacDougall, R. (1903). The structure of simple rhythm forms. Psychological Monographs. American Psychological Association. pdf

BibTeX reference:

@article{macdougall1903structure,
  title={The structure of simple rhythm forms.},
  author={MacDougall, R.},
  journal={Psychological Monographs},
  year={1903},
  publisher={American Psychological Association}
}