In the poem, what is the blacksmith's swinging of his sledge compared to?

The poem says of the blacksmith:

You can hear him swing his heavy sledge,With measured beat and slow,Like a sexton ringing the village bell,When the evening sun is low.

The village bell is the church bell. In the past, this was often the only way people who could not afford clocks or watches knew what time it was. The sexton was the groundskeeper for the church, whose duties included looking after the church's physical well-being and ringing its bell.
Therefore, the swinging of the blacksmith's sledge, slow and rhythmic, is like the ringing of the church's bell. This is a pleasing sound image, as most people enjoy the ringing of church bells and connect them with spirituality. The blacksmith's sledge's ringing pleases us, becoming part of the rhythm of village life. This accords with the sentimental idealizing of the blacksmith and his work that is the theme of the poem. The simple village blacksmith—hardworking, religious, and loyal to the memory of his mother—is meant to tug at our heartstrings and become a role model for us all.