*In the chapter at the beginning of Webster's dictionary devoted to accent it is stated that these words are accented on the last syllable because by derivation the root rather than the prefix receives the accent. This “great principle of derivation” often fails, it is admitted. We have indicated above a natural reason why it cannot fail in the cases mentioned. The voice would be incapable of accenting easily the unimportant prefix in such a word as _ac-cuse′,_ for instance.
In words of three syllables, the accent is usually on the first syllable, especially if the second syllable is weak and the last syllable no weaker if not indeed stronger. Thus we have _pe′-ri-od, per′-son-ate, It′-aly,_ etc.
If for any reason the second syllable becomes stronger than either the first or the last, then the second syllable must receive the accent and the syllable before it is usually strengthened. Thus we have _i-tal′-ic,_ and there is a natural tendency to make the _i_ long, though in _Italy_ it is short. This is because _tal_ is stronger than _ic,_ though not stronger than _y_. The syllable _ic_ is very weak, but the obscure _er,_ or, _ur_ is still weaker, and so we have _rhet′-or-ic_. In _his-tor′-ic_ the first syllable is too weak to take an accent, and we strengthen its second syllable, giving _o_ the _aw_ sound.
It will be seen that in words of two or more syllables there may be a second, and even a third accent, the voice dwelling on every other syllable. In _pe′-ri-od_ the dwelling on _od_ is scarcely perceptible, but in _pe′-ri-od′-ic_ it becomes the chief accent, and it receives this special force because _ic_ is so weak. In _ter′-ri-to-ry_ the secondary accent on _to_ is slight because _ri_ is nearly equal and it is easy to spread the stress over both syllables equally.
The principles above illustrated have a decided limitation in the fact that the value of vowels in English is more or less variable, and the great “principle of derivation,” as Webster calls it, exercises a still potent influence, though one becoming every year less binding. The following words taken bodily from the Greek or Latin are accented on the penult rather than the antepenult (as analogy would lead us to accent them) because in the original language the penultimate vowel was long: abdo′men, hori′zon, deco′rum, diplo′ma, muse′um, sono′rous, acu′men, bitu′men; and similarly such words as farra′go, etc. We may never be sure just how to accent a large class of names taken from the Latin and Greek without knowing the length of the vowel in the original,——such words, for example, as _Mede′a, Posi′don_ (more properly written _Posei′don_), _Came′nia, Iphigeni′a, Casto′lus, Cas′tores, etc_.