[Daniel Kirkwood to Barnard]

Daniel Kirkwood was an American mathematician who used mathematics to derive theories based on the work of astronomers. He was the first to suggest that meteor showers were composed of debris from comets. He also developed a theory known as Kirkwood’s Gap. This theory asserts that the gaps in the main asteroid belt between Jupiter and Mars were caused by the heavier gravitational pull of Jupiter. He alludes to the theory in this letter, stating there are additional Jovian satellites yet to be discovered.