The observations of the Prâtisâkhya are most minute. For instance in Sûtra 465 certain words are enumerated which if they stand at the beginning of a line must always lengthen their final vowel. One of them is arka which must become arkâ. This applies to the whole of the Rig-veda. But the author of the Prâtisâkhya adds ‘there is an exception in the case of Bharadvâga that is in the hymns of Bharadvâga in the sixth Mandala. And so it is. In the sixth Mandala (VI. 68 9) all our MSS. have arka unchanged before devâya.