MR. hood's theory. 



29 



formed of, it appears that earth enware, or bright tinned iron pipe require 

 to be of greater size than cast ii'on ones, to produce a hke effect. 



The cause of the circulation of hot ^ater in pipes has been yariously 

 explained, and the theory laid down by Tredgold, before alluded to, was 

 considered the most rational and complete. The correctness of that 

 opinion has, however, lately been called in question by Mr. Kood, who 

 obsers'es that the theory of Tredgold Tvill not account for the cumulation 

 of water, imder all ckcumstances, and ever}' yariety of form of the ap- 

 paratus ; and as the cause of motion must be the same in all cases, 

 any explanation which will not apply universally must necessarily be 

 erroneous. 



In order to explain his theory ]\Ir. Hood proceeds, let ns suppose 

 heat to be apphed to the boiler A," in the accompan^-ing diagram. " A 

 dilatation of the volume of the water takes place, and it becomes Hghter ; 

 the heated particles rising upwards through the colder ones, which sink 

 to the bottom by their greater specific gra^-ity, and they in their turn 

 become heated and expanded hke the others. This intestine motion con- 

 tinues imtil all the particles become equally heated, and have received as 

 much heat as the fuel can impart to them. But as soon as the water in 

 the boiler begins thus to acquire heat, and to become hghter than that in 

 the opposite vessel B, the water in the lower horizontal pipe d, is pressed 

 with a greater weight at z than at y, and it therefore moves towards 

 A with a velocity and force equal to the difference in pressm-e at the 

 two points y and z. The water in the upper part of the vessel B would 

 now assume a lower level, were it not that the pipe c furnishes a fresh 

 supply of water from the boiler to replenish the deficiency. By means of 

 this unequal pressiure on the lower pipe, the water is forced to circulate 

 through the apparatus, and it continues to do so as long as the water in 

 B is colder, and therefore heavier, than that which is in the boiler ; and 

 as the water in the pipes is constantly parting mth its heat, both by 

 radiation and conduction, while that in the boiler is as continually re- 

 ceiwlng additional heat from the fire, an equahty of temperatm'e can never 

 occur, or else f it did, the circulation would cease." 



