He is the author of several popular works on astronomy.

He is the author of several works concerning foreign policy and the international economy.

He was the author of several works, which, however, are lost.

He was the author of several works on Christian belief.

He was author of several works relating to military history.

He was the author of several scientific, educational and religious works.

He is the author of several works in this discipline.

He was an author of historical works, especially active from 1898-1930.

He was the author of several works relating to American history.

He was the author of several works on diplomatic history.