148 Recent Advances of Mechanical Science. 



the profession, seeing that the structure has to be one fit to 

 contend with all the duties a railway bridge has to perform as a 

 girder, with this tremendous difference, that the railway girder 

 may be, and often is, made ten times the weight of the load it 

 carries, while a ship has to contend with the waves — no unim- 

 portant addition to the strains brought to bear on the structure 

 common to both constructions. She has also to carry her 

 engines, boilers, coal, stores, and water, and the gross weight of 

 all these and the material used in her structure is only about 

 one-third of the weight she carries, i.e.,, her displacement. 

 Also, in the construction of ships and their fittings the work of 

 an architect comes into play, not only in giving character to the 

 external appearance of the structure, but in all the details which 

 constitute the comfort of home life, giving comfortable accom- 

 modation to man and beast night and day, with warmth and 

 ventilation under difficult circumstances ; while many other 

 difficulties which surround ship construction are not necessary to 

 be considered in structures on land, and are all the more difficult 

 to design and arrange for, owing to the necessarily restricted 

 space available. 



From an engineering pomt of view, therefore, a vessel, besides 

 being a girder, has to be designed to carry an enormous amount 

 of power, in many cases more than one-horse power per ton of 

 the gross tonnage of the vessel, and the exercise of this must 

 not unduly make a vibration in the structure. On this subject 

 much could be said. Theory has had to follow practice in this 

 as in many other things. To deal with the peculiarities of 

 vessels which have only manifested themselves by experiments 

 with different forms and areas of propellers, different pitches 

 and speeds of screws, would fill up an evening by itself. 



Few people think, even those who are daily conversant with 

 the construction of mills, what the differences of the conditions 

 of working are; say of the engines in some of our big local mills, 

 and at sea. Probably there is not one of these where over 

 2,000 indicated H.P. are used. In steamers six to eight times 

 this amount is not at all unusual. These mill engines are run 



