On IiicruskUioiis of Boilers tmng Sea- Water. 285 



for their purpose. If Peclet's formula for calculating this surface is to 

 be trusted, those chests on board the West India Mail Steamship 

 La Plata, and some of the British and North American Company's 

 packets are iVth to Ath of the size that would be efficient. When these 

 brine chests, regenerators, or heat economizers, thei'efore, are made 

 with a sufficient amount of surface, so that abundance of water can be 

 supplied to and discharged from the boilers, with little loss of heat, 

 then there will be no incrustation of boilers, and a probable saving of 

 from 12 to 13 per cent, of their fuel. Peclet's formula^ or Professor 

 Eankine's reduction of it, which gives the probable amount of surface 

 required for a difference of temperature of 140° between the feed and 



the discharged water, at r^th square foot per lb. of brine discharged 



per hour, becomes under the same circumstances, and when the quantity 



of brine discharged is equal to the quantity of water evaporated, ^rS^ 



square foot of surface per lb. of water evaporated per hour. The intro- 

 duction of Dr. Joule's spiral wires to the system will probably render 

 less surface efficient. This amount of discharge and surface, it is 

 expected, will prevent incrustation, and save nine-tenths of the heat 

 at present lost. 



On the Density of Steam. By Prof. W. J. Macquorn Rankike. 



It has been known for some time that the Density of Steam deviates 

 from the laws of the perfectly gaseous condition ; and deviates more 

 and more as the pressure increases. 



A formula for deducing the density of a vapour from its pressure, 

 temperature, and latent heat, was first deduced from the Mechanical 

 Theory of Heat, by Professor Clausius, in 1849. 



Professor Rankine, in the absence of precise experimental data, made 

 use of a formula substantially identical with that of Clausius, to com- 

 pute tables of the volume and density of steam for practical use, which 

 have been published in his work On Prime Movers. 



Experiments have for some time been in progress by Mr. Fairbairn 

 and Mr. Tate, on the density of saturated steam at various boiling- 

 points, part of which were communicated to the British Association in 

 September, 1859. In the following table, the results of these experi- 

 ments are compared with those of the theory, computed by the aid of 

 the tables of volume and density before mentioned. 



