Complex sentence: After Watson treats him, Davy writes on a chalkboard a series of numbers, which, once deciphered, are co-ordinates pointing to Ardnamurchan Lighthouse on a Scottish coastline.
Split into simpler sentences: After Watson treats him, Davy writes on a chalkboard a series of numbers. They also decipher Davy's number, which they find are co-ordinates pointing to Ardnamurchan Lighthouse on a Scottish coastline.