MECHANICS AND USEFUL ARTS. 33 



of a large quantity of liontcd water in a boiler is now well understood, 

 and. such as it is, it increases as the square of the diameter, so that, 

 in a boiler of a given length, the elements of weakness and danger 

 are collectively related to the cube of the diameter. External heat- 

 ing surface may be provided for in a number of smaller vessels, as in 

 the retort boilers by Mr. Dunn, but these are of the water-tube fam- 

 ily, which, heretofore, has been found subject to choking with the 

 solid matter deposited by the water. 



Internal Heating Surface. Next are the boilers with internal heat- 

 ing surfaces. Internal lire tubes were in use in steam boilers in the 

 last century, and they were applied within a cylindrical barrel by the 

 Cornish engineers, among whom was Trevithick, who employed both 

 the straight Hue and the return line, and who made the fireplace 

 within the Hue. The Cornish boiler, in this form was improved by 

 Mr. Fairbairn and the kite Mr. Hetherington, who added another 

 fire tube, thus making the two-lined boiler now so extensively em- 

 ployed in Lancashire. The two Hues, although somewhat smaller 

 than the single Hue, afforded a greater extent of heating surface, be- 

 sides securing increased regularity in firing. The principle of sub- 

 dividing the liame and heated products of combustion, so as to obtain 

 greatly increased heating surface within a barrel of given diameter, 

 w;ft fully carried out in the multitubular boiler invented by Neville, 

 of London, in 1826, employed by Seguin, in France, in 1828, and 

 subsequently in the Liverpool and Manchester locomotives, from 

 which it has been handed down to the present practice of engineers. 

 Not since Watt's time, however, has the evaporative power of a 

 square foot of heating surface been increased, the improvement in 

 the plan of steam boilers being that, chieHy, of inclosing a greater 

 extent of surface within a given space. The heating surface, in the 

 boilers of the Great Eastern .steamship, is equal to the entire area 

 of her vast main deck ; that in the Adriatic measures more than three- 

 fourths of an acre, while the Warrior and the Black Prince have, in 

 their boilers, '2,,'<()0 square yards of surface of tubes, the aggregate 

 length of which is more than five and one-half miles. 



Uljjedions to Midtitubidar Boilers. But it is only where, as in 

 steamships and in locomotive engines, the dimensions and weight 

 of boilers must be the least possible, that the multitubular arrange- 

 ment is even to be tolerated. It is costly and subject to rapid decay. 

 In steamships, especially, the life of multitubular boilers is compara- 

 tively short. The boilers in her Majesty's vessels of war are found 

 to last but from five to seven years ; those of the West Indian 

 Eoyal mailships last, according to Mr. Pitcher, of Northfield, six 

 years only, and those of the Dover and Calais packets, taking the 

 testimony of the former mail contractor, Mr. Churchward, need to be 

 renewed every three and a half or four years. On land, multitubu- 

 lar boilers, working under constant strain, and, in most cases, as 

 constantly concentrating a saturated solution of sulphate of lime, are 

 nearly out of the question for the purpose of manufactories, although 

 there are instances of their employment, even in spinning mills. A 

 boiler rated at 40 nominal horse-power will ordinarily evaporate 60 

 cubic feet of water per hour, or upwards of 100 tuns of water per 

 woik of CO hours. And feed water containing as much as 40 grains 



