Branches of linguistics. Synchronic vs diachronic approaches to the language study. Lexicology – ‘the science of the word’

Sound-interchange is the gradation of sounds occupying one and the same place in the sound form of one and same morpheme in various cases of its occurrence.
Historical causes of sound-interchange:
ablaut (vowel gradation), i.e. a change of one to another vowel accompanying a change of stress, e.g. to ride – a road; to bear – a burden; to bite – a bit etc.;
umlaut (vowel mutation), i.e. a partial assimilation to a succeeding sound, e.g. full – to fill, a tale – to tell etc.;
consonant interchange, e.g. to speak – a speech, to bake – a batch, to live – a life etc.
20. Semantics as a branch of linguistics, its aims and basic notions. Semasiological and onomasiological perspectives of the English lexicon.
Lexical semantics is the systematic study of meaning-related properties of words.
The term semantics for the subdiscipline of linguistics concerned with the analysis and description of the so-called ‘literal‘ meaning of linguistic expressions was introduced in 1897 by the French linguist Michael Bréal, the founding father of present-day semantics.
That made a serious and largely successful attempt to introduce semantics into European linguistic work. And, once the Swiss linguist Ferdinand de Saussure had made the linguistic sign the cornerstone of his influential theories, semantics was here to stay in European linguistics
Onomasiology (Gr. ònomasía ‘name, designation’, logos ‘study’) is a subdiscipline of lexical semantics that studies the word meaning in the direction ‘from the concept – to a sound form (or forms)’. Thesauruses are compiled according to onomasiological principles.
Semasiology (Gr. sēmasia ‘signification, meaning’ and lógos ‘study’) is a sundiscipline of lexical semantics concerned with the studies of the word meaning in the opposite direction: ‘from the sound form – to its meaning (or meanings)’.
The distinction was introduced by the Austrian linguist Adolf Zauner in 1903 his study on the body-part terminology in Romance languages. Both disciplines can be treated diachronically and synchronically
The term semasiology was introduced by Christian Karl Reisig in 1825 in his Lectures on Latin Linguistics.
The objective of semasiology is to expose and explain meanings signified by word sound forms and to demonstrate the difference between these meanings.