At six o'clock, Miss Howard, disguised as Alfred Inglethorp, enters the chemist's shop, with her story about a dog, obtains the strychnine, and writes the name of Alfred Inglethorp in John's handwriting, which she had previously studied carefully. 
Miss Howard spent three weeks carefully studying John's handwriting. 
mnli hypothesis: Miss Howard spent three weeks carefully studying John's handwriting.  premise: At six o'clock, Miss Howard, disguised as Alfred Inglethorp, enters the chemist's shop, with her story about a dog, obtains the strychnine, and writes the name of Alfred Inglethorp in John's handwriting, which she had previously studied carefully. 