Engravings of the Temple of Serapis are familiar to historians of geology. The Temple, which showed evidence of having at some time sunk below sea level, has been made into an icon used to represent Lyellian uniformitarianism. Since Lyell's use of the 1820 engraving which appeared as the frontispiece to the first and subsequent editions of his Principles of geology (see Figure 1), accounts of the Temple have been central in debates regarding uniformitarian theories in geology. I However, in 1847 the seventh edition of Lyell's Principles was published with a new frontispiece ( Figure 2) depicting a new view of the Temple. This illustration was drawn in 1836 by an artist who used a camera lucida -a relatively new instrument designed to reflect landscape images on paper for more accurate tracing. Why did Lyell change frontispieces?The answer is directly related to a paper by Charles Babbage on the Temple of Serapis, also published in 1847, in the Journal ofthe Geological Society ofLondon, for which this new illustration was specially prepared. Babbage developed a theory of land subsidence and elevation which centred on the effects of a subterranean heat source on gradual and uniform changes in the earth's crust. To illustrate his theory, Babbage used a special technique of mechanically reproducing drawings to represent changes in the relative level of land and sea, and he used his calculating engine to work out mathematical relations of the effects of subterranean heat on rock expansion and contraction.The theory Babbage put forward regarding the uniform processes of geological change -of the uniformity of the actions of land subsidence and elevation -was different from Lyell's own, which was altered in later editions of the Principles as a consequence of Babbage. Babbage is most familiar as the "pioneer of the computer", and his work in geology has been only marginally explored by historians. His geological studies were few: approximately half-a-dozen papers and a thin book in total within a corpus of over seventy articles and six books, most of which relate to his interests in mathematics. But Babbage's geological ideas arose out of his broader concerns over reform in scientific practice and ways of representing scientific knowledge. His interests in political economy and manufactures, in the mechanization of print and illustration, in accurate drawing and graphic design (particularly relating to the construction of his calculating engine), all shaped the way Babbage thought 0073-2753/98/3603-0299/$2.50