The work must, however, be regarded as in no sense whatever a finished production, though I hope it will be found not only accurate but also readable. In fact, I could not possibly have found time to rewrite the whole in the form in which I should like to have presented it for publication; so that the reader is requested to remember, if he desires to find fault, that the nonremoval of many defects whose correction would have required large changes, was the condition under which alone the book could have appeared. Still, I should not have allowed it to be published had I not been assured by competent judges that in spite of its necessary imperfections it is calculated to be useful.