That is a more than slightly alarming passage!

Honestly lots of the Necronomicon is various quantities of alarming. 

It really is, though she's somewhat more concerned with threats to humanity as a whole.

"Why," Joseph hisses under his breath, "did the Mad Arab include that formula to summon Yog-Sothoth. Under what circumstance could that possibly seem like a good idea." 

"Possibly they were right to term him mad," she murmurs back.

"Tch. At least it's temporally constrained..." He frowns, then checks to see when it would have to happen. Then relaxes slightly. "And the world didn't end last week, so we're safe for the moment." 

"Are there other potential times?"

"Yes, but not until the stars--or some subset of them--form the exact same configuration relative to Earth again." 

"We should perhaps note the relevant astronomical circumstances, so we'll know when to expect potential repeats."

He nods and writes that down. 

And further, then. Anything else time-sensitive?

Nothing so earth-shaking. There are some instructions for doing nasty things that could be going on this very moment and they wouldn't know but might like to stop if they were?

She sighs and tries to figure out if it's obvious how to tell if the nasty things are going on.

Well, with this formula for mind transference, if someone suddenly starts acting more like someone else than themself, that's a clue. If Cthulhu is unleashed people might start having strange dreams. Aside from that...not really. 

They'll keep an ear out, then. And mail Emma a warning.

When they've gotten everything they can from the Necronomicon, they bid Professor Armitage goodbye and leave. 

"I'd like to go back to the post-master's; it seems important, that Emma and Hiram at least know what to look for if there's local problems..."

"Yes, that seems wise." 

And off, to take care of that.

And then to a bookstore to pick up Nereus's anatomy book. 

Nereus and Helen are waiting for them back at the hotel.

"People don't like Innsmouth much," Helen reports. "They're insular and look funny." 

"They didn't really get suspicious of us for asking, but yeah they were kind of mean about the locals."

"One guy I talked to said the superstitious stuff was probably nonsense and it was probably just race prejudice. And there was nothing wrong with that. Nothing wrong with that! Who admits they think that, north of the Mason-Dixon line???" 

"Better than the guy I talked to who kept going on about voodoo..."