The astonishing level of enthusiasm over the birth of the new prince goes far beyond the pleasure that people naturally feel for an attractive young couple who have had a healthy child. If there is any truth at all to these estimates in the North American media that trinkets and other bric-a-brac, and even increased numbers of tourists, will produce hundreds of millions of pounds for the British economy, the answer lies not just in normal goodwill and the effusions of the most strenuous monarchists.