In contradistinction to the plagiarists Vitruvius expresses his deep gratitude to the men of the past who have written books, and gives lists of his authorities,[873] and declares that “the opinions of learned authors ... gain strength as time goes on.”[874] “Relying upon such authorities, we venture to produce new systems of instruction.”[875] Or, as he says in discussing the properties of waters, “Some of these things I have seen for myself, others I have found written in Greek books.”[876] But in describing sun-dials he frankly remarks, “I will state by whom the different classes and designs of dials have been invented. For I cannot invent new kinds myself at this late day, nor do I think that I ought to display the inventions of others as my own.”[877] He also gives an account of a number of notable miscellaneous discoveries and experiments by past mathematicians and physicists.[878] Also he sometimes repeats the instruction which he had received from his teachers. Like Pliny a little later he thinks that in some respects artistic standards have been lowered in his own time, notably in fresco-painting.[879] But also, like Galen, he once admits that there are still good men in his own profession besides himself, affirming that “our architects in the old days, and a good many even in our own times, have been as great as those of the Greeks.”[880] He describes a basilica which he himself had built at Fano.[881] [Sidenote: Machines and Ctesibius.] Vitruvius’s last book is devoted to machines and military engines. Here he makes a feeble effort to introduce the factor of astrological influence, asserting that “all machinery is derived from nature, and is founded on the teaching and instruction of the revolution of the firmament.”[882] Among the devices described is the pump of Ctesibius of Alexandria, the son of a barber.[883] He had already been mentioned in the preceding book[884] for the improvements which he introduced in water-clocks, especially regulating their flow according to the changing length of the hours of the day in summer and winter. Vitruvius also asserts that he constructed the first water organs, that he “discovered the natural pressure of the air and pneumatic principles, ... devised methods of raising water, automatic contrivances, and amusing things of many kinds, ... blackbirds singing by means of waterworks, and _angobatae_, and figures that drink and move, and other things that have been found to be pleasing to the eye and the ear.”[885] Vitruvius states that of these he has selected those that seemed most useful and necessary and that the reader may turn to Ctesibius’s own works for those which are merely amusing. Pliny more briefly mentions the invention of pneumatics and water organs by Ctesibius.[886] [Sidenote: Hero of Alexandria.]