20(3 REPORT — 18G0. 



minimum temperature compatible with a given amount of cooling surface, or the 

 greatest quantity of heat extracted from the products of combustion, before their 

 escape to the atmosphere. The soot forming usually inside of boilers will not be so 

 injurious in this arrangement, as it will fall down through the external crevices, and 

 also between the spiral and the centre boilers into the furnace below, and be thrown 

 overboard with the ashes. 



This spiral coil and all the heating surfaces will keep more clear of flue dust than 

 usual, and will consequently be more efficient in that respect, as well as save the 

 usual trouble and loss by sponging experienced in the ordinary tubular boilers at sea. 

 Also as the products of combustion must pass off at the rate of at least 7 feet per second 

 in this as in the ordinary boilers, it will take upwards of 14 seconds from the time 

 it leaves the furnace till it arrives at the top of the boiler ; whilst if the boiler were 

 of the ordinary tubular type, it would pass in about two seconds along the whole 

 heating surface of the boiler ; the gas has therefore seven times more time to give 

 out its heat, and its revolving tendency will not admit of the same strata of gas 

 passing along the passages after it is cooled down, as is the case with the ordinary 

 boiler, but will bring the hot products of combustion usually occupying the centre 

 of the tubes of a tubular boiler in contact with the cooling surfaces, and reduce the 

 whole products of combustion to one temperature before entering the chimnev. 



In cleaning the salt or sludge out of these boilers, the man- and sludge-hole doors 

 are taken off the top and bottom (and the hose with fresh water may be played down 

 through from the top, and the refuse run out at the bottom). The man in charge 

 can also pass down through the whole boiler, the dimensions necessary for this pur- 

 pose being made the minimum and maximum of the various compartments of the 

 boiler; and are specially constructed to maintain to the engines steam at much 

 higher pressure than usual, in order to admit of a much larger amount of expansion 

 to be developed by the engines, which are all on the double cylinder expansive prin- 

 ciple. The constructors are now making the boilers for three steam-ships on this 

 principle, two of which are for carrying Her Majesty's mails on the Pacific between 

 Valparaiso and Panama (as described by the writer at the meeting of the British 

 Association at Leeds) ; and it has long been his desire to be able to construct boilers 

 for marine purposes without stays, and with no surface exposed to the collapsing 

 tendency, which in so many cases has been the cause of loss of life aboard of steam- 

 ships. The boilers now described have no large flat surfaces and no stays, the whole 

 tendency of the pressures being to inflate the boiler plates, and, if possible, to give 

 them a stronger form ; the smallest diameter is large enough to give access to the 

 men in charge, and the largest diameter 34 inches and -} thick, — dimensions that can 

 carry several hundred pounds pressure on the square inch before rupture could take 

 place. Such a form the writer adopts, with great satisfaction to himself, as a con- 

 structor sending machinery abroad, where the usual form of boiler gives him consi- 

 derable anxiety. In comparing the construction of this boiler with that of the 

 ordinary tubular one, in the latter angle-iron ribs and stays now compose a large 

 portion of the weight and expense ; contribute no heating surfaces ; and if one stay 

 breaks, which is not an uncommon occurrence, the next is placed in great danger ; 

 and if it gives way, the whole may follow in rotation, and a serious accident be the 

 result. In the former boiler, however, the plates may be reduced to a very small 

 amount of thickness by tear and wear before explosion could be expected. 



Having thus described the objects of the spiral boiler, it may not be out of place 

 to give the following statement of the comparative evaporative power and temperatures 

 of the gases in the furnace and chimney of the spiral boiler, with three of the ordi- 

 nary types of boiler now in general use. 



Fig. 1 is a vertical section of the cylindrical spiral boilers as fitted on board the 

 Pacific Royal Mail Company's steam-ships * San Carlos' and 'Guayaquil,' by 

 Messrs. Randolph, Elder, &c. Fig. 2 is a sectional plan of the same, taken near the 

 level of the water-line in fig. 1. Fig. 3 is a vertical elevational view of the same — 

 the exterior casings which surround the circumferential vertical tubes (and which 

 are shown in figs. 1 and 2) being in this view removed. 



It will be seen from these figures that there are in these boilers 21 tubes in all, 

 viz. 19 circumferential vertical tubes, 1 central and 1 spiral tube. 



The three types experimented upon were, first, a common cvlindrical land boiler 



