This subject will be considered under two heads: 1. The efficiency of the screw propeller, compared with the paddlewheel. 2. The advantages or disadvantages attending its use (p. 539)--The first question involves the consideration not merely of the effect produced, but also of the proportionate power absorbed in producing that effect (p. 539)--Observations on the slip of the paddlewheel and screw in the ‘Great Western’ and ‘Archimedes,’ respectively (p. 540)--The result of the comparison being, that with similar areas the screw will meet with at least equal, if not a greater resistance, and will consequently slip as little as or less than the ordinary paddle-board (p. 542)--Refutation of the common error, that the action of the screw is a very oblique one, and that it imparts a considerable rotatory motion to the water (p. 543)--Description of the action of the screw (p. 543)--The result of the experiments is that, as compared with the ordinary paddlewheel of sea-going steamers, the screw is, both as regards the effect produced and the proportionate power required to obtain that effect, an efficient propeller (p. 548)--Objections to the use of the screw propeller (p. 548)--Answered seriatim (p. 549)--Statement of the principal advantages peculiar to the use of the screw (p. 552)--The result of these enquiries is a strong and decided opinion in favour of the screw propeller being adopted in the ‘Great Britain’ steam-ship (p. 557). LIST OF ILLUSTRATIONS. _ENGRAVINGS._ Portrait (engraved with permission of Messrs. Graves and Company by T. O. Barlow, from a portrait by J. C. Horsley, R.A., in the possession of Mrs. Brunel). ....._Frontispiece_ PLATE. ....._to face_ PAGE I. The Clifton Suspension Bridge.....49 II. The Hungerford Suspension Bridge.....59 III. The Royal Albert Bridge at Saltash. General View......171 IV. The Newport, Windsor, and Chepstow Bridges. Elevations and Sections.....206 V. The Royal Albert Bridge at Saltash. Elevations and Sections......218 _WOODCUTS._ FIG. .....PAGE 1. Maidenhead Bridge (longitudinal section).....174 2. Bourne Viaduct (outline elevation).....181 3. Ivybridge Viaduct (the same).....182 4. Landore Viaduct (the same).....184 FIG. PAGE 5. St. Pinnock Viaduct (outline elevation, transverse section, and plans of piers).....187 6. Experimental Girder (transverse section).....193 7. Girder on South Wales Railway (the same).....194 8. Girder on Eastern Bengal Railway (the same).....195 9. Cumberland Basin Swing Bridge (transverse section of girder).....196 10. Windsor Bridge (transverse section of truss).....200 11. Chepstow Bridge (the same).....208 12. Saltash Bridge (the same).....219 13. Transverse Section of the ‘Great Britain’ Steam-Ship.....256 14. Copy of a Sketch in a Letter on Iron Ships.....259 15. Transverse Section of the Cradles and the Launching Ways of the ‘Great Eastern’ Steam-Ship.....347