HOW TO DRAW A STRAIGHT LINE 41 Going back to the Scott Russell linkage we can see that the short guide GG could, in principle at least, be replaced by any of the linkages already described in this chapter. A Watt linkage for example gives a very close approximation to a straight line near to its centre of movement and would be quite suitable. Cheby- shev experimented with arrangements, including those with his linkages. An engine using this arrangement was displayed at the Vienna Exhibition in 1873. Comments on this engine were reported in Engineering on 3 October (p. 284). The motion is of little or no practical use, for we can scarcely imagine circumstances under which it would be more advan- tageous to use such a complicated system of levers, with so many joints to be lubricated and so many pins to wear, than a solid guide of some kind; but at the same time the arrangement is very ingenious and in this respect reflects great credit on its designer. So, by the time of Scott Russell, in practice any gains that could