Learning Japanese in the End of the Edo Era

I am reading Ernest Satow's book "A Diplomat in Japan". It is very interesting.
Ernest Satow was the first interpreter between English and Japanese. When he became an interpreter, there wasn't anyone who could speak both English and Japanese in England or in Japan. At that time English diplomats and the Japanese government might have communicated through a mixture of English, Dutch and Japanese.
Recently I, myself, am learning English, so I am interested in how he learned Japanese in such circumstances. He wrote in his book as follows.

…Towards the end of October we induced the colonel S. R. Brown, and to allow us to engage a native “teacher,” … A “teacher,” it must be understood, does not mean a man who can “teach.” In those days, at Peking and in Japan also, we worked with natives who did not understand a word of English, and the process by which one made out the meaning of a sentence was closely akin to that which Poe describes in the Gold Beetle for the decyphering of a cryptograph. … So Takaoka began to give me lessons in the epistolary style. He used to write a short letter in the running-hand, and after copying it out in square character, explain to me its meaning. Then I made a translation and put it away for a few days. Meanwhile I exercised myself in reading, now one and now the other copy of the original. Afterwards I took out my translation and tried to put it back into Japanese from memory. (p56) http://goo.gl/6fNre

He might have happened to learn Japanese using direct method. In this quotation Satow wrote "Takaoka … explain to me its meaning". I am interested in how Takaoka, his Japanese teacher, explained the meaning without using English.
Satow wrote in his book about many episodes in which he talked with Japanese warriors, "Samurai", so he could speak Japanese well, I guess. But he was an interpreter for diplomacy, and he was supposed translate documents. It was very difficult for him.

… The Mikado’s decree to his vizier the Kwambaku delegating the conduct of foreign affairs to the Tycoon, a short document of only three lines, was enclosed in it. … It was proud night for me when I displayed my knowledge of written Japanese in the presence of the French minister, whose interpreter, M. Mermet, even could not read a document without the assistance of his teacher. (p154) http://goo.gl/6fNre

Mermet, a French interpreter, couldn’t read a Japanese diplomatic document, and Satow prided himself on reading just a short document of only three lines. I can imagine how poor the ability of interpreters was at that time.
As a side note Satow's English is modern and I can read it easily, but it is difficult for me, a Japanese, to read Japanese writings from the same age. Japanese writing has changed greatly from at that time to nowadays. How interesting it is!