Although it might seem gimmicky at the outset, virtually delivering his speech saved about 15 tonnes of carbon dioxide - what it would have taken had Prince Charles and his staff actually flown to the summit.

What's deliciously interesting (to me, anyway) about this is the method they used to deliver the speech: using a stage-magic method from the 19th century known as the Pepper's Ghost Illusion. Below is a modern-day example:

Credit for the illusion was popularly given to Professor John Henry Pepper, a 'performing' chemist at the Royal Polytechnic on Regent Street. However, it was purchased for some £500 from Henry Dircks, who had originally named the illusion as the "Dircksian Phantasmagoria". Dircks had no small amount of trouble selling his idea to theatres, as adopting his method of bringing ghosts to the stage involved the laborious reconstruction of the theatre around the illusion mechanisms. Projected images of the ghostly actors would need to be produced from below the stage:

The effect his illusion would produce was astounding, and bringing ghosts to the stage in manners more supernatural would be highly sought after. Alas, as summarised by Jim Steinmeyer in "Hiding the Elephant":

Not having any success selling his invention, in 1862 Dircks took it to the Royal Polytechnic, where it was purchased by Professor Pepper. Pepper originally utilised the illusion on Christmas Eve in a performance of Charles Dickens' "The Haunted Man". And although Pepper had intended to use the play as a technical demonstration and a lead-in to a subsequent explanation of the ghostly illusion, it turned out he had a hit on his hands. Later, he would license it out to other theatres and music halls and the name of the illusion would be mis-credited by the press, despite Pepper's efforts to have Dircks's name included.

It also turns out that the Prince of Wales of that era, Prince Albert, was introduced to the illusion first-hand by being invited into the contraption that would make him appear as a ghost to others.

These days, the most famous place you can witness the Pepper's Ghost illusion is in the Haunted Mansion ride at the Disney theme parks - just look for the ghostly ballroom. Professor John Henry Pepper also authored "The Boy's Playbook of Science", available freely from The Internet Archive, which details more optical illusions that Professor Pepper pleasurably performed at the Polytechnic.