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You have to use a number of letters which are easily divisible by a few numbers so that there aren't just random spaces at the end of the line where the x's are in this case. That wouldn't work when you tried to encrypt it. One of the problems with the transition cipher is that you can discover it by checking for anagrams. With modern technology, this is very easy, with numerous anagram-finding websites. The message for decoding is easy to solve with the trans-poser tool from the Cipher toolkit. The number of letters is 192, so the number of columns has to be a number divisible that can multiply into 192, and the number of columns is unlikely to be too large. It therefore has to be 4,6,8 or 16. (It won't be 1 and 2 because it doesn't really rearrange the message at all). After a few attempts, you can see that it's 8. The message is this:
The Rabbit hole went straight on like a tunnel for some way and then dipped suddenly down so suddenly that alice has not a moment to think about stopping herself before she found herself falling down what seemed to be a very deep well(x).