Consider we have a helicoid:

A hellcold is defined by @: D - R', where X - P C08 0. = r sine. 1=0 and D is the region where 0 3 0 $ 2x and 0 $ r $ 1 (Figure 7.4.2). figure 7.4.2 The hallcold x - / cool (1, 0, 0)