During the days when ships were powered by sail, the captains log documented everything that happended during the
day. As sickness could spread rapidly on a ship,there were often times where the number of sailers that were ill exceeded the space provided in the log to record their names. During these times, the excess names of the
sick were recorded in the
next column, which was reserved for the weather conditions of the
day. Thus, it was not unusal for an ill sailor to be listed "under the weather".