Before revision: The Portuguese scientist and explorer Joao de Castro was the first to research and report such an inconsistency, in 1538, and attributed it to the ship's guns, while also describing local variations caused by rock formations, and obtaining 48 rigorous recordings of magnetic deviation across the Atlantic and Indian oceans"When on August 5, 1538, D. João de Castro decided to determine the latitude of Mozambique, found the cause that dictated the astonishing uneasiness of needles; noted the deviation of the needle, discovering it 128 years before Dennis Guillaume (1666) of Nieppe, which is recorded in History of Sailing as if he were the first to know about this phenomenon.
After revision: The explorer Joao de Castro was the first to report such an inconsistency, in 1538, and attributed it to the ship's guns, while also describing local variations caused by rock formations, and obtaining 48 rigorous recordings of magnetic deviation across the Atlantic and Indian oceans"When on August 5, 1538, D. João de Castro decided to determine the latitude of Mozambique, found the cause that dictated the astonishing uneasiness of needles; noted the deviation of the needle, discovering it 128 years before Dennis Guillaume (1666) of Nieppe, which is recorded in History of Sailing as if he were the first to know about this phenomenon.