STEAM NAVIGATION. 



upwards and pass backwards and forwards through a level of the 

 water nearer its surface, finally terminating in the chimney. More 

 heating surface is thus obtained with the same capacity of boiler. 

 It is found in practice, that the most efficient parts of the flue- 

 surface for the generation of steam, are those which are horizontal 



Fig. 7. 



and at the upper parts of the flue, and the least efficient those 

 which are horizontal and at the lowest part, the efficiency of the 

 vertical sides being intermediate. 



Since the flues are liable occasionally to become choked with 

 soot and ashes, it is necessary that their magnitude shall be 

 sufficient to allow a boy to enter them for the purpose of cleaning 

 them. 



31. Tubular marine boilers are constructed on a principle pre- 

 cisely similar to that of locomotive boilers, described in a former 

 Tract. The flame and gaseous products of combustion, issuing 

 from the furnace at a very elevated temperature, pass through a 

 great number, sometimes several hundred, tubes of iron or brass, 

 of about three inches diameter, which traverse the boiler below the 

 level of the water in it, so that before they enter the chimney their 

 temperature is reduced to a comparatively low point, the heat 

 they thus lose being taken up by the water surrounding the tubes. 



Flue boilers have the advantage over tubular boilers in being 

 cheaper and more durable. With the same evaporating power 

 they are however one-third larger and heavier, and consequently 

 occupy a greater portion of the* tonnage, and produce, other 

 things being the same, a proportionally greater displacement, the 

 latter condition augmenting the resistance, and therefore either 

 diminishing the speed, or increasing the consumption of fuel. 

 134 



