This antiphon is one of my favourites amongst Byrd's antiphons it's a prayer for help and the coming of peace. Its text is taken from the Vulgate (Matthew 8: 25–26) – the verses recounting how Jesus stilled the tempest in the Sea of Galilee:
25 Et accesserunt ad eum discipuli ejus, et suscitaverunt eum, dicentes: Domine, salva nos: perimus.
Byrd uses several musical devices to achieve the effects he desired as you listen you'll hear him use a descending scale at 'perimus' (we perish) to portray the sinking of the disciples while the serenity of the musical phrase he uses for 'tranquillitatem' and poignancy of the antiphon's final cadences never fail to move me. It's sung below by The Cardinall's Musick conducted by Andrew Carwood. Enjoy :-).