Page:The Sign of Four.pdf/21

12 my part, and I have mentioned it to no one.'

'It is simplicity itself,' he remarked, chuckling at my surprise—'so absurdly simple that an explanation is superfluous; and yet it may serve to define the limits of observation and of deduction. Observation tells me that you have a little reddish mould adhering to your instep. Just opposite the Wigmore Street Office they have taken up the pavement and thrown up some earth, which lies in such a way that it is difficult to avoid treading in it in entering. The earth is of this peculiar reddish tint which is found, as far as I know, nowhere else in the neighbourhood. So much is observation. The rest is deduction.'

'How, then, did you deduce the telegram?'

'Why, of course I knew that you had not written a letter, since I sat opposite to you