FROM C. J. MONRO, Esq.

                                                           Hadley, Barnet, 10th September 1871.

     . . . Of your own things, the Classification of Quantities and the Hills and Dales, are all I have read to much    purpose. Nor them either, you may say, if I go on to ask why you say that "in the pure theory of surfaces there is     no method of determining a line of water-shed or ater-course, except as therein is excepted, that is in    page 6? Why does not this determine them? to wit—

 
 

     Or if this does determine them, how does it resolve itself into "first finding," etc.?

     I am glad you like Strutt on sky-blue. You see he sees his way now to a new theory of double refraction. Looking    at your old letter again, I don't quite see the force of either of your objections to space of more than three    dimensions. First, you ask if we can think some of the dimensions and not others, then which? Surely one might    answer, that depends—depends namely on your circumstances—on circumstances which in your circumstances    you cannot expect to judge of.

     "I can easily believe," as Darwin would say, that before we were tidal ascidians we were a slimy sheet of cells   floating on the surface of the sea. Well, in those days, the missing dimension, and the two forthcoming ones  respectively, kept changing with the rotation of the earth,—we now know how, but could not guess then. So,   now, the missing dimension or dimensions, if any, might be determined by circumstances which we could not tell   unless we knew all about the said dimension or dimensions.