WATER TUBE BOILERS. 607 



French navy), and the Shann boiler to be described hereafter, and 

 torpedo boilers : in this last case the connection seems at first sight 

 to be with the spiral tubes, but by looking at the Yarrow boiler it 

 will be seen that the lower water drums, though cylindrical in 

 form, are really identical wath the " water jackets" or " waterlegs," 

 which distinguish all boilers of this class. (2) Boilers consisting 

 of coiled water tubes : It is not clear how spiral boilers are con- 

 nected with such boilers as are called sectional by their manufac- 

 turers. If, however, we flatten a spiral into a sort of gauche 

 zigzag figure we get sometliing resembling the elements of a 

 Belleville boiler ; and it is not a far step from this, with its series 

 of pairs of headers, to the Babcock &; Wilcocks boiler, with its 

 sections, each containing two continous headers, and forming a con- 

 necting link, as it were, between the first type and this latter type, 

 since in turn each section might be considered as a separate boiler. 



The writer first had his attention turned to these boilers some 

 fourteen years since bj' a relative who had patented a new type of 

 marine boiler, which was to be a combination of the water tube and 

 fire tube boiler, but it was not until July, 1891, when he had occa- 

 sion to travel from Melbourne to Marseilles in the M.M.S. Poly- 

 nesien that he fully learnt the advantage of boilers of the water 

 tube ty'pe for marine purposes. On the voyage the weather was 

 unusually adverse, a heavy head sea running until the Red Sea 

 was reached, and as the captain was disappointed in getting coal 

 at Aden the chief engineer was forced to use every jjossible 

 economy until Menzaleh was reached ; there sufficient coal was 

 taken on (15 tons) to enable the ship to go through the canal. 

 The writer was horrified to hear that one of the modes of econo- 

 mising fuel was to dispense with the evaporator used for making 

 up the feed and make up with sea water. The engineer-in chief 

 explained that owing to their system of mixing milk of lime with 

 the hot feed the salts in the sea water were almost all deposited in 

 the separator, which is an important part of all marine boilers of 

 the Belleville class. 



This explanation did not agree with the writer's preconceived 

 ideas of the chemistry of this subject, so he made a point when 

 at the ship building and repairing yards of the company at La 

 Ciotat to get further information. 



He was there shown water tubes which had been removed from 

 marine boilers after six or seven years' service, and the iron (they 

 were of lapwelded wrought iron) was coated internally by black 

 oxide, Avhich formed a smooth coating. This was due to the 

 formation of very greatly superheated steam on the surface where 

 the water was in contact with very hot metal. This coating not 

 only protects the pipe, but it will be readily seen that where steam 

 is being so rajiidly generated there is not much danger of incrusta- 

 tion or deposit, and consequently salt water feed can be used with 

 impunity. 



