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Dorn, Harold; MacClellan, James E. (2006). Science and Technology in World History: An Introduction. Johns Hopkins University Press. ISBN 0-8018-8359-8.

Aiyangar, Sakkottai Krishnaswami; S. Krishnaswami A. (2004). Ancient India: Collected Essays on the Literary and Political History of Southern India. Asian Educational Services.ISBN 0-8018-8359-8.

Prasad, Prakash Chandra (2003). Foreign Trade and Commerce in Ancient India. Abhinav Publications. ISBN 81-7017-053-2.

Lowe, Robson M.H. (1951). Encyclopedia of British Empire Postage Stamps ed. 2 hal.5-71. London.

Mazumdar, Mohini Lal (1990). The Imperial Post Offices of British India. Calcutta: Phila Publications.

Mote, Frederick W.; John K. Fairbank (1998). The Cambridge History of China. Cambridge University Press. ISBN 0-521-24333-5.