104 BULLETIN 173, U. S. NATIONAL MUSEUM 



spheres or wide shallow vessels of various and often fantastic shapes 

 covered with flat metal plates. All of them were heated over open 

 fires or simple stone stoves in which the heat of the fire was applied 

 to only a small section of the surface of the vessel. They seem to 

 have been constructed of copper. 



The recent and continuous development of the steam boiler begins 

 with the boilers used to supply steam at low pressure to the at- 

 mospheric steam engines of Newcomen. It is true that before New- 

 comen Thomas Savery had constructed boilers for the high pressures 

 that his mine pumps (pulsometers) required, but the difficulty of 

 maintaining them tight and his lack of success with boilers in general 

 were largely responsible for the abandonment of the Savery engine. 

 Before Savery and Newcomen, soapmakers, brewers, and other trades- 

 men using evaporators and cookers had developed boilers and fur- 

 naces with considerable skill and thought. Brick and masonry 

 settings in which flues and passages in the setting were used to con- 

 duct the flame and hot gases over the surface of the boiler shell 

 had been evolved. The boilers themselves were simply cylindrical 

 straight-sided and flat-bottomed vessels open at the top and usually 

 made of copper. To adapt them as steam generators they Vvere 

 covered with a sheet of metal, often just a sheet of lead. Tlie boiler 

 illustrated in the engraving of the Newcomen engine of 1712 (see 

 above) was a boiler of this type with a hemispherical, domed top 

 forming a steam chamber above the cylindrical part of the boiler. 

 The top was joined to the lower part in a wide flange, which over- 

 hung the sides at the top and formed the top of a flue that sur- 

 rounded the entire lower part of the boiler. Gradually this type of 

 boiler was rendered more suitable for steam power purposes. 

 Wrought-iron plates and riveted joints were used (about 1725), the 

 sides and bottom were made concave for stiffness, and internal stays 

 were employed. In its improved form the boiler resembled a hay- 

 stack and was often called the haystack boiler. It was sometimes 

 constructed with a central domelike firebox and an internal helical 

 flue. 



To increase the heating surfaces of the boiler James Watt, about 

 1780-1790, designed the toagon type of boiler, which is practically 

 an elongated haystack boiler with flat ends, somewhat resembling 

 a deep rectangular wagon body with an arched top. These boilers 

 had concave sides, which with the adjacent brickwork of the setting 

 formed flues along each side of the boiler. The grates were at the 

 forward end and beneath the boiler, and the hot gases passed under 

 the concave bottom to the back, returned along one side of the boiler, 

 and then passed back again along the opposite side. Later Watt 

 added a square flue through the center of the boiler and caused the 



