Caloric half-years

Figure 5 Caloric summer half-year insolation following Milankovitch s definition plotted for the latitudes of 40° N, 60° N, and 80° N. Caloric half-years are periods of equal duration where each day of the summer half-year receives more insolation than any day of the winter half-year.

That matter was not indefinitely divisible but had an atomic or molecular structure was q working hypothesis for most scientists from the eighteenth century onwards. There was a minor reaction towards the end of the nineteenth century when a group of physicists who professed a positivist philosophy pointed out how indirect was the evidence for the existence of atoms, and their objections were not finally overcome until the early years of this century. If in retrospect, their doubts seem to us to be unreasonable we should, perhaps, remember that almost all those who then believed in atoms believed equally strongly in the material existence of an electromagnetic ether and, in the first half of the nineteenth century, often of a caloric fluid also. Nevertheless those who contributed most to the theories of gases and liquids did so with an assumption, usually explicit, of a discrete structure of matter, llie units mi t be named atoms or molecules (e.g. Laplace) or merely particles (Young), but we will follow modern convention and use the word molecule for the constituent element of a gas, liquid, or solid. 

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