How to read "Till a’ the seas "?

a' = ?

Context:

The poem
O my Luve's like a red, red rose
That’s newly sprung in June;
O my Luve's like the melodie
That’s sweetly play'd in tune.

As fair art thou, my bonnie lass,
So deep in luve am I:
And I will luve thee still, my dear,
Till a’ the seas gang dry:

Till a’ the seas gang dry, my dear,
And the rocks melt wi’ the sun:
I will luve thee still, my dear,
While the sands o’ life shall run.

And fare thee weel, my only Luve
And fare thee weel, a while!
And I will come again, my Luve,
Tho’ it were ten thousand mile.