Complex sentence: In 1913-1914, when the Boston Athenæum employed the architectural firm of Bigelow and Wadsworth to expand the building, the fourth and fifth floors were set back so as not to disrupt the symmetry of the façade.
Split into simpler sentences: In 1913-1914, the Boston Athenæum employed the architectural firm of Bigelow and Wadsworth to expand the building. The fourth and fifth floors were set back so as not to disrupt the symmetry of the façade.