Floo & Fireplace Communication Into/Out of Hogwarts

[corinthum]

No: HPFGUIDX 81638

Snazzzybird wrote:

> I've been wondering about fireplace communications to and from 
> Hogwarts, specifically with regard to the incident in OoP when 
Sirius 
> was speaking to Harry in the Gryffindor Common Room fire. An 
instant 
> after he left the fire, Umbridge's hand appeared, clutching at the 
> space where his hair had been. Sirius was speaking from #12 
Grimmauld 
> Place. If Umbridge had gotten hold of his hair -- well, okay, he 
> probably would have been able to get free. But what if he hadn't? 
If 
> he had withdrawn his head from the fireplace with her hand still 
> entangled in his hair, would she have been able to "trace the call" 
> so to speak? Might she have popped out into the kitchen of Order of 
> the Phoenix headquarters -- a location which does not exist for 
her, 
> because she has not been told about it by the secret keeper?

An interesting question.  On the surface, I would say no.  Let's 
forget the Fidelius Charm for a second.  So far, we've seen that 
enunciation of your destination is necessary when traveling by floo 
powder.  Harry ending up in Knockturn Alley due to mispronunciation 
is movie contamination, but it is still emphasized in the books that 
each person must clearly state a destination before stepping into the 
flames.  So it would seem to me that Umbridge could not reach a 
partcular place by floo if she couldn't state it's address.

However, what would have happenned in the Umbridge/Sirius situation?  
I don't have my books with me, so I can't check the exact wording of 
the scene.  But were Sirius and Umbridge ever in the fire 
simultaneously?  If I remember correctly, Sirius disappeared and a 
moment later Umbridge's arm appeared.  The question is, is it 
possible for two people to "dial in" to the same fire at the same 
time, or would on receive a busy signal?  Also, the Weasleys always 
travel one at a time, which suggests that each open "line" can only 
transport one person.  Could a floo hitch-hiker travel on someone 
else's open line if he or she managed to find it out in floo space?  
Also, are floo highways two-way streets, or one-way?  We know that 
you can travel backwards if you never fully entered the fire (as 
Diggory, Sirius, and Harry do), but can a person completely in floo-
space travel out of that same path?  

Umbridge's actions seem to imply that she expected to be able to find 
something in the Gryffindor fire.  Was she trying to grab Sirius, or 
simply looking for tangible evidence that he had been there a moment 
earlier?

warning: complete speculation ahead
My theory is that a fire can only be open to one incoming traveler at 
a time.  This explains why the Ministry of Magic would need many 
fireplaces to accomodate all their employees and visitors.  It would 
also solve the problem of collisions, which you would think would be 
pretty common in popular destinations if anyone could floo in at any 
time.  It also explains why the Weasleys don't save money on floo 
powder by joing hands and having the first traveller haul the rest 
through.  I also think that the Floo Network is a one-way system.  If 
(going against my previous theory) Umbridge was able to grab Sirius, 
I think she would have hit a wall when she reached the Grimmauld 
Place fire; in order to open that path to herself, she would have to 
add more floo to her office fire and state the new destination name.
Therefore, I don't think Umbridge was trying to grab Sirius so much 
as she was trying to find some immediate proof as to who was using 
the fire a moment before. 

-Corinth