SIR - None of your usually vociferous green' correspondents has yet provided any answer to my earlier poser of why hydro-electricity is not being promoted as an effective address to their climate-change concerns.

Is this a secret they don't want discussed? Or maybe they do just want an excuse to put up taxes.

So, in the light of that deafening silence, here's another question: If unnecessary air travel is such a bogeyman to the green lobby, how come they've not yet started a vigorous campaign against religious pilgrimages?

Just think of all that completely wasted fuel and the excess carbon dioxide created by those millions of pilgrims jetting off every year to the likes of Lourdes, the Holy Land and Mecca, from all points of the compass.

If the green lobby claims science to promote its cause, then that same science will also confirm that the basis of all pilgrimage travel is completely vacuous and, therefore, should be eliminated.

But maybe there are too many soft and suggestible votes at stake with that. So much for green' principles!

Graham Hoyle, Kirkbourne Grove, Baildon