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Continuous Transmission Frequency Modulated (CTFM) ultrasonic sensors can be used to recognise plants. The echo from a plant is modelled as an acoustic density profile. Classification based on features extracted from the echo is more robust than classification based on the echo. Potential applications for this sensing system include landmark navigation and plant sensing for selective spraying of agricultural chemicals.

Communications regarding this column should be addressed to Mrs. Cheney, Peabody Library School, Nashville, Term. 37203. Mrs. Cheney does not sell the books listed here. They are available through normal trade sources. Mrs. Cheney, being a member of the editorial board of Pierian Press, will not review Pierian Press reference books in this column. Descriptions of Pierian Press reference books will be included elsewhere in this publication.

It is now forty years since there appeared H. R. Plomer's first volume Dictionary of the booksellers and printers who were at work in England, Scotland and Ireland from 1641 to 1667. This has been followed by additional Bibliographical Society publications covering similarly the years up to 1775. From the short sketches given in this series, indicating changes of imprint and type of work undertaken, scholars working with English books issued before the closing years of the eighteenth century have had great assistance in dating the undated and in determining the colour and calibre of any work before it is consulted.