X-rays will prove to be a hoax.

Wireless (telegraphy) is all very well but I’d rather send a message by a boy on a pony!

The wireless telegraph is one of the most wonderful inventions the world has ever seen. I think it will be of great commercial use some day and as I have seen it demonstrated on the ship in which I have just arrived I can say that it is very marvelous indeed.

Trust you will avoid the gigantic mistake of alternating current. (writing to Niagara Falls Power Company)

Heavier-than-air flying machines are impossible. (1895)

I have not the smallest molecule of faith in aerial navigation other than ballooning, or of the expectation of good results from any of the trials we hear of. (1896)