Huygens's 1688 Report to the Directors of the Dutch East India Company on the Measurement of Longitude at Sea and the Evidence it Offered Against Universal Gravity

When Christiaan Huygens prepared the 1686/1687 expedition to the Cape of Good Hope on which his pendulum clocks were to be tested for their usefulness in measuring longitude at sea, he also gave instructions to Thomas Helder to perform experiments with the seconds-pendulum. This was prompted by Jean Richer's 1672 finding that a seconds-pendulum is 1 1/4 lines shorter in Cayenne than in Paris. Unfortunately, Helder died on the voy¬age, and no data from the seconds-pendulum ever reached Huygens. He nevertheless did receive data from his clocks on the return-voyage from the Cape of Good Hope to Texel. When he first calculated the ship's course according to these data, it appeared to have gone straight through Ireland. He then tried introducing a correction to the data, based on an idea he had previously entertained as a possible explanation of Richer's finding: he corrected the observed time to com¬pen¬sate for a reduction in the effect of gravity from the Poles to the Equator resulting purely from the Earth's rotation. His newly calculated course convinced him that this rotational effect is the sole source of any variations in gravity with latitude. This paper examines Huygens's correc¬tions to the data and his reasoning from the new course to the conclusion that nothing else causes a variation in gravity. In the process, we show that Huygens had cogent empirical reasons to reject Isaac Newton's theory of universal gravity, which predicted a some¬what larger variation in gravity.