Wall of serpents

The Mathematics of Magic was probably the greatest discovery of the ages - at least Professor Harold Shea thought so. With the proper equations, he could instantly transport himself back in time to all the wondrous lands of ancient legend. But slips in time were a hazard, and Shea's magic did not always work - at least, not quite as he expected . . . The Wall of Serpents is the third in L. Sprague de Camp and Fletcher Pratt's much-loved Compleat Enchanter series.