32 ANNUAL OF SCIENTIFIC DISCOVERY. 



and describing in particular a form of boiler invented by Mr. Joseph 

 Harrison of Philadelphia. We give the substance of this paper un- 

 der its several heads : 



Evaporation to Surface. The rate at which heat may be trans- 

 mitted through an iron boiler plate, without injury to its substance, 

 has never been precisely ascertained. About 70,000 units of heat 

 per hour, equal to the evaporation in that time of one cubic foot of 

 water from b'0, is believed to be the utmost per square foot of plate 

 of ordinary thickness. But in order to approximately apply the 

 whole heat of a furnace to the purposes of evaporation, a much larger 

 area of heating surface, per unit of work done is requisite. Watt 

 fixed the proportion of one square yard of heating surface per cubic 

 foot of water evaporated per hour, and this has been sanctioned by 

 modern practice. But the average depth, or, in other words, the 

 thickness of the stratum, of water thus boiled away is only one and 

 one-third inches per hour, one forty-fifth-inch per minute, or one 

 twenty-seven thousandth inch per second, over the whole heating 

 surface. From ten to twelve seconds are thus occupied in vaporizing 

 a coucke of water, no thicker than a single leaf of the paper upon 

 which books are commonly printed. If, in proportion to the evapo- 

 ration, an insufficient extent of heating surface be provided, there is 

 not only a direct waste of heat, the products of combustion escap- 

 ing at a temperature corresponding, perhaps, to that of incande- 

 scent iron, but the furnace plates maybe burnt. Notwithstanding 

 the active convection of heat in water, an intense Ham* directed 

 against the sides or roof of a boiler furnace, will, in time, crack or 

 blister the iron. It is not certain that this result occurs from the in- 

 ability of the metal to transmit the heat, for it is more likelv that, 



* ' 



under vigorous vaporization, the gravity of the liquid water (and it 

 is its gravity only that brings it to the heating surfaces) is insufficient 

 to bear down effectively against the rising volumes of steam. If, by 

 powerful mechanical means, the water could be constantly maintained 

 m contact with the heating surfaces, it is possible that the rate of 

 evaporation upon a given area could be increased without injury to 

 the plate. In the hardening of anvil faces and of steel dies, the re- 

 quisite rapidity of cooling is obtained, not merely by immersion in 

 water, but by its forcible descent, in a strong jet, upon the heated 

 rnetaL 



External heating Surface. Under the conditions, however, of or- 

 dinary practice, no restriction of the heating surface is permissible. 

 This surface, is sometimes that of the exterior only of the boiler, but 

 it is more usual, and on most accounts preferable, to dispose it inter- 

 nally by means of fire boxes, flues, or tubes. The external surface of 

 a boiler can only be increased by increasing its length or its diameter, 

 or by increasing both of these together. Plain cylindrical boilers 90 

 feet, and in one or two instances 104 feet in length, have been em- 

 ployed, but even apart from any consideration of the great amount 

 of space which they occupy, they are mechanically objectionable, and 

 they are now no longer made. In increasing the external surface of a 

 boiler by enlarging its diameter, it is weakened exactly in proportion 

 to the increase, the bursting pressure, lor a given thickness of plates, 

 being inversely as the diameter. The danger attending the presence 



