The Upanishads, Part 1 (SBE01), by Max Müller, , at sacred-texts.com
1. The Gâyatrî 3 (verse) is everything whatsoever here exists. Gâyatrî indeed is speech, for speech
sings forth (gâya-ti) and protects (trâya-te) everything that here exists.
2. That Gâyatrî is also the earth, for everything that here exists rests on the earth, and does not go beyond.
3. That earth again is the body in man, for in it the vital airs (prânas 1, which are everything) rest, and do not go beyond.
4. That body again in man is the heart within man, for in it the prânas (which are everything) rest, and do not go beyond.
5. That Gâyatrî has four feet 2 and is sixfold 3. And this is also declared by a Rik verse (Rig-Veda X, 90, 3):--
6. 'Such is the greatness of it (of Brahman, under the disguise of Gâyatrî 4); greater than it is the Person 5 (purusha). His feet are all things. The immortal with three feet is in heaven (i. e. in himself).'
7. The Brahman which has been thus described (as immortal with three feet in heaven, and as Gâyatrî) is the same as the ether which is around us;
8. And the ether which is around us, is the same as the ether which is within us. And the ether which is within us,
9. That is the ether within the heart. That ether in the heart (as Brahman) is omnipresent and unchanging. He who knows this obtains omnipresent and unchangeable happiness.
44:3 The Gâyatrî is one of the sacred metres, and is here to be meditated on as Brahman. It is used in the sense of verse, and as the name of a famous hymn. The Gâyatrî is often praised as the most powerful metre, and whatever can be obtained by means of the recitation of Gâyatrî verses is described as the achievement of the Gâyatrî. The etymology of gâyatrî from gai and trâ is, of course, fanciful.
45:1 The prânas may be meant for the five senses, as explained in Kh. I, 2, 1; II, 7, 1; or for the five breathings, as explained immediately afterwards in III, 13, 1. The commentator sees in them everything that here exists (Kh. Up. III, 15, 4), and thus establishes the likeness between the body and the Gâyatrî. As Gâyatrî is the earth, and the earth the body, and the body the heart, Gâyatrî is in the end to be considered as the heart.
45:2 The four feet are explained as the four quarters of the Gâyatrî metre, of six syllables each. The Gâyatrî really consists of three feet of eight syllables each.
45:3 The Gâyatrî has been identified with all beings, with speech, earth, body, heart, and the vital airs, and is therefore called sixfold. This, at least, is the way in which the commentator accounts for the epithet 'sixfold.'
45:4 Of Brahman modified as Gâyatrî, having four feet, and being sixfold.
45:5 The real Brahman, unmodified by form and name.