in earthquakes weak spasms precede the worst convulsions

An example of an extended metaphor in science writing:

"This eruption, probably, was the first indication of those great volcanic disturbances which ensued exactly one month afterwards; for it seems to be a very general occurrence in earthquakes, that weak spasms precede the worst convulsions."

Charles Darwin, 1840, On the connexion of certain volcanic phenomena in South America; and on the formation of mountain chains and volcanos, as the effect of the same powers by which continents are elevated. (Transactions of the Geological Society of London)