The Shadow Constant 1st edition

Someone is keeping watch over the machine. Should it ever have been built? 
 
The lure of the find behind the loose hearthstone at Hazelton House was too great. The schematic excited Evan, Reenie and Ivy for different reasons but it is Kayla who actually begins building it. Her ability to focus to the exclusion of all else is just one of the effects of her Asperger's.

The scribbled initials E. W. were merely a curiosity until it's discovered they belong to Eli Whitney. But strange footprints and disturbing visitors let Evan know that the theft of his sister's prize schematic isn't just a coincidence.

It becomes obvious that the generator has no apparent power source. And Eli Whitney was only the first to die because of the machine.

Soon they realize their enemies are much bigger than they had imagined and ?the threat the device poses could topple an empire. The secret now threatens those at Hazleton House.
        
Who is trying to stop them?