1. UFOs, Spaceships, whatever E.Ts use (we’ll probably never know). He may have wanted to stay behind and let his own species leave without him.

2. He went for men too, though only slapping them. It’s been psychologically proven that being in an atmosphere different from your own makes you go funny in the head (When Neil and his gang landed on the moon, they started laughing insanely, and it wasn’t necessarily because of what they had achieved, but because the unfamiliar atmosphere was doing things to their heads. Jack was also known to be laughing insanely) There’s hundreds of reasons why he may of chosen to go specifically for women, maybe one of them was because he found them vulnerable/easier to terrorize and doing this amused him. Also, Most know the astronauts can’t walk on the moon due to the fact humans are too powerful for the moon’s gravity, so they have to jump, and they could jump quite easily up to 30 ft. (Jack was reported to have usually been jumping up to 25 ft) Maybe Jack’s own gravity was different, which is why he was leaping all over the place, maybe his planet is bigger and has a higher gravitational pull, being reared in this environment would make Jack experience the earth as a low-gravity world, again like we do the moon.

3. Just because of Atmosphere and Gravity, doesn’t mean his environment has to be too different from ours, and also just because of his basic human appearance, doesn’t mean he has to come from a similar environment. And why does everyone think aliens have to be these green skinned brown eyed creatures? And no, Jack wasn’t described as too human looking. yes, he had four limbs and a pair of eyes (which were said to glow like two fiery balls, suggesting he’s from somewhere nocturnal) but he was always described as ‘unhuman-like’ and ‘Hideous’ or simply just ‘strange looking’. Maybe Jack didn’t look human, perhaps his appearance was fake, a disguise/costume he made to try and look as human as possible.

As for the ‘blue-fire’, it probably wasn’t fire at all. (No singes or scorch marks were reported on the victims) probably blue phosphor (only explanation I can find). Many of our species on earth have bio-luminescence, which means they can radiate light through the actions of some of the compounds in their metabolism. (One example would probably be the glow worm) Spring-Heeled Jack (if he were alien) could of had the ability to create bio-electricity, which means his breathing system could of been electrically charged, which would explain why his victims would have fits after he breathed the blue gas into their faces. Species like the electric-eel can do this and can create a paralyzing shock of up to 600 volts.

Also from all that I’ve read, the sightings of him went past the human life-span (or the adult life-span at least). This destroys the mad-inventor theory (which would be tough theory anyway considering springs can break your ankles) because obviously Jack couldn’t of started as a child. The ‘Inventor’ would of had to of been an adult, and that adult would of become too old to do it eventually. You could say that maybe some people had picked up on the gadgets or perhaps there was a son who carried out Jack’s work if he were an inventor, but then why would they go around attacking people anyway? Any inventor would rather find a way to make money out of it than to go around terrorizing people just for fun, and if it was someone mentally retarded, then they wouldn’t of had the intelligence to build springs like that,

Another fact is that the Victorians didn’t have time to go prancing around attacking people, even at night time, they needed long sleep for all the work they had to do, as the Victorian era wasn’t as lavish as today, only the richest could spend time on hobbies, and again if it was an Inventor, like anyone, they’d want to make money out of their inventions. Also why haven’t scientists and modern inventors come up with springs capable of launching you 25 ft into the air. There was also the case of World War II, where the Germans put springs in the heels of their boots, and ended up breaking their ankles, so there’s all evidence of springs and spring-like substances not being able to launch you too far without breaking something.
So, I’m with the alien hypothesis, because it’s the only thing that seems to fit logically.

And they’d have gotten away with it, too…if it weren’t for those meddling kids and their dog!