Item 220 - Letter from Peter Barlow

3 pp.

Woolwich - Further to WW's steam train enquiry: 'I beg to say that I have not made the comparison you allude to, with the experimental results, because those results involve in them the velocity with which the train reaches the plane, which velocity alone would carry the train some way up the plane without engine powers the experimental speeds therefore are greater than they ought to be for the comparison in question the formula having reference only to the mechanical impediment of the slope. It will of course be observed that although the momentum might actually carry the train to the top of the plane without engine powers - the original velocity has to be acquired again so that this ascent proves no mechanical gain of power. The question of the disadvantage of the plane must therefore rest on its own ground and the way to proceed is to enquire what additional load would on a horizontal plane produce the same resistance as the inclined plane'. PB gives an example of a ratio which accounts for various resistances - such as friction, air and the atmosphere.