Tuesday 19 July 1836

July 19^th Today Harriet has been much better. She slept well in the cot, and passed the greater part of the day on deck. In the course of the day we spoke the Mount Stuart Elphinstone on her voyage to Calcutta. In the evening Harriet saw for the first time the phosphorescence of the sea: on the breaking of a wave, or on the water being otherwise agitated, sparkles of great brilliance were abundant. The quantity of light emitted by each is probably equal to that of a glowworm; on a dark night I have seen the ship quite illuminated by this means – the phenomenon was first perceived by us in the Bay of Biscay & I am told that when at the Equator the intensity of the light will be much greater. The cause of this extraordinary appearance is said to be animalculae.