Long-Term Intervals

“But the intervals have been examined all the way back to the eighteenth century. The first one uncovered ran from 1798 to 1813.”

An “interval” is simply the elapsed time from an important low to an important high or from a high to a low. It is called an interval because we ignore the fluctuations in between. It doesn’t matter what its overall trend was while the interval was in progress, or how many times it changed, or whether it ends higher or lower than the level at which it began.