Lotty, with Teutonic phlegm, was calmly eating bread and currant wine, for the jelly was still in a hopelessly liquid state, while Mrs. Brooke, with her apron over her head, sat sobbing dismally.
The scale is based on the temperatures which control the physical state of water, with zero degrees Celsius being the temperature that ice melts. In other words, when water changes from a solid to a liquid state.