America's mourner in chief

It is a grim part of the president's job to be mourner-in-chief, comforter-in-chief.

President Barack Obama did so after the shooting of Gabby Giffords and the murder of six others; he did so after the cinema shooting in Aurora, Colorado; and again after the murders of the children in Newtown.

But this was the first time he has had to speak at such a service after an act of terrorism.

As we don't know who carried out this act, or why, it is not surprising that he had few words for the bomber or bombers themselves. He called them "small stunted individuals" and promised that they would face justice.

Technically, it wasn't one of his most polished speeches. It had the feeling of a first draft, perhaps a little too heavy with imagery of marathon-running and sunshine. But that is hardly important.

His main purpose was to make America feel good about itself, and to make it see itself as the sort of nation that wouldn't lapse into fear in the face of such an attack.

No hint of being at war or under siege or standing firm in dark times. Instead a stress on Boston's spirit, with a few jokes thrown in, and an insistence that America cannot be, will not, be cowed.

As ever, Mr Obama expressed America's essential nature as a country that comes together, a people who help each other, who are there for each other.

"We know that somewhere around the bend, a stranger has a cup of water, around the bend, somebody is there to boost our spirits," he said.

"On that toughest mile, just when we think that we've hit a wall, someone will be there to cheer us on and pick us up if we fall."

Something like this is his most common refrain, a very familiar message, that America is about community, a country which looks after its own.

In describing his US, he is also spelling out what he thinks it should be.

But he also knows what he does not wish it to become. The president had to come here, had to help Boston heal, but he does not want the country dwelling and brooding on this one terrible attack.