MLA

Backus, Isaac, 1724-1806. Collected papers. Box 7, folder 18. 1749 February-March. https://collections.library.yale.edu/catalog/33554015.

APA, 6th edition

Backus, Isaac, 1724-1806. (["Box 7, folder 18"])(1749 February-March)[Collected papers]. https://collections.library.yale.edu/catalog/33554015.