What are isothermal lines?

Some articles on isothermal lines:

Humboldtian Science - Humboldt’s Equilibrium
... In order to graphically represent this continuity Humboldt developed isothermal lines, which denoted isotherms ... These isothermal lines functioned in the general balancing of forces in that isothermal lines preserved local peculiarities within a general regularity ...

Famous quotes containing the word lines:

    He does smile his face into more lines than is in the new map with the augmentation of the Indies.
    William Shakespeare (1564–1616)