TRANSACTIONS OF SECTION G. 713 



have a proper supply of the former to the surfaces through which the heat of com- 

 bustion is passed. The fallacy of making steam by " foaming " or " vapour circula- 

 tion " for boilers is pointed out, and Mr. 0. Wye Williams's table of the relative 

 effects of water, steam, and air in contact with the plates of a boiler which are 

 heated, is quoted. 



The natural action of circulation in boiling is well known to consist, first, in the 

 directly upward motion of the heated portions of water, and the vertical escape of 

 the steam, and second, in the colder portions of the water seeking the coolest means 

 of descent, when possible, undisturbed by opposing currents. 



A correctly designed boiler will provide for the free action of these simple prin- 

 ciples, which are contravened more or less by boilers of the ordinary marine type, and 

 by all those having horizontal or horizontally inclined water tubes, of which several 

 examples are named. None but a boiler of vertical water tube design satisfies indis- 

 pensable conditions. . 



2. The form of the boiler introduces the question of strength, in which there are 

 two important factors, viz. (a) the best form for resisting pressure, and (b) equal 

 distribution of the strains due to expansion by heat, (a) High pressures demand 

 diminished areas, with of course an absence of flat or large-rending surfaces or large 

 diameters, and thus boilers of water tube design are required. Boilers of ordinary 

 marine type, which have large external diameters, also are defective in exposing 

 large surfaces to loss of heat by radiation, and so increasing the strains caused by 

 unequal expansion, (b) The force exerted by expansion is considerable, and is 

 enough to prove that the even distribution of heat by proper circulation and design 

 has a large influence upon the strength and wear of boilers. Cases of boilers 

 which suffered damage from expansion unequally distributed are quoted, and the 

 author refers to various measurements of this force given bv Professor W. Allen 

 Miller ('Chem. Phys.,' 4th ed., p. 261), and by J. Milton ('"Strength of Boilers,' 

 Proc. I.N.A.) 



3. In considering facility of construction and repair, the author refers to the 

 drawings illustrating his paper, and recapitulates the defective points of various de- 

 signs, which have come up in the preceding remarks, and he contends for simple 

 operations in making, and for the minimum of different kinds of parts composing a 

 boiler. 



It is being generally' understood and admitted that boilers ought to be worked 

 exclusively with fresh water, and with precautious against corrosion such as are 

 suggested in the Report of the Admiralty Committee on Boilers or in the author's 

 paper on 'Boiler Corrosion' (British Assoc, September, 1876), it becomes unneces- 

 sary to provide for access to all the internal parts. Facility of removal of parts for 

 repair is therefore all that is nece.-sary under these circumstances. 



5. On the System of Dredging usually employed in the United States. 



By Robert Briggs. 



6. On a Nciv Ship-raising Machine. By Thomas A. Dillon. 



An oval-shaped floating canvas mattress larger and wider than the ship to be 

 raised, and having a deep curtain of strongly netted and roped canvas brailed up 

 all round, is lowered fore and aft over the wreck, when masts are supposed to have 

 been cut way, down below low water as far as convenient. The canvas curtain, 

 well weighted and chained, is by suitable means allowed to fall so that when it 

 reaches the bottom the ship lies in the centre of an oval bell-tent. An ordinary 

 suction pump is put in action at first ; the result is that the vacuum created in the 

 upper portion of the bell-tent induces the water on the bottom to try and fill it. 

 This it cannot do : it merely compels the bottom of the tent or flexible diving-bell 

 to grasp the wreck. A running rope or wire cable running string which had been 

 rove round the mouth of the bag is hauled taut. 



The suction pump now ceases, and the air pump is worked. The water is 

 driven out of the bell by air pressure, and when an amount of water equal to the 

 weight of the ship is expelled, the ship rises in the grasp of this octopus. 



