SECT, in.] CONDENSATION OF STEAM. 125 



was essentially a short cylinder placed on its base, and terminating in a hemi- 

 spherical head. 



224. Watt's Boilers. A rectangular form was adopted by Mr. Watt for the 

 lower portion of the boiler ; the upper part he made half a cylinder : the bottom 

 was made concave, but the sides flat. For low pressure steam a boiler may be 

 made abundantly strong of this form, and it affords a little more surface without 

 materially increasing the space the boiler occupies. Making the bottom concave 

 towards the fire also may cause the sediment to settle in the angles instead of 

 immediately over the fire. In large boilers a flue was formed through the middle 

 of the boiler, so as to be covered by the water within. 



It was justly remarked by Mr. Watt, that the sole object of the arrangement of 

 his boilers "was to economise the fuel as much as possible. It is not the 

 shallowness or depth of the boiler that produces this effect ; but the making of the 

 boilers of such a shape that the air which passes through the fire shall be robbed of 

 almost all its heat before it can make its escape." 1 Mr. Watt assured Dr. Thomson 

 that this object is very well attained by the construction he had adopted, and it 

 undoubtedly is so. 



225. When a boiler of a rectangular plan (see Plate i.) is used, the relations 

 of the length, width, and depth to obtain the necessary quantity of surface and of 

 capacity are easily found, when it has no internal flues ; and it is doubtful whether 

 any advantage is gained by such flues or not. The following is an approximate 

 rule for the purpose. 



RULE. Take the capacity of the boiler for water, and divide it by the quantity of 

 bottom surface, (art. 221.) the result will be the depth of water. 



Multiply together the bottom and side surface for fire and flue, (art. 221.) and 

 divide the product by twice the capacity for water, less the area of the bottom 

 surface, and the result will be one of the dimensions of the bottom. 



Divide the bottom surface by the dimensions found, and it gives the other. 



Example. To find the proportions of a boiler for an engine of 12 horse power, 

 the capacity for water being 12' 2 cubic feet for each horse power. 



In this case 12 x 12'2 = 146 - 4 = the capacity of the boiler for water ; and the 

 bottom surface 5 x 12 = 60 feet, hence 



60 

 the depth of water. 



146 ' 4 = 2-44 feet, 



Dr. Thomson's Annals of Philosophy, vol. vii. p. 173. 



