Lagging of Steam-Pipes. 519 



In order to test this equation we have taken two thin 

 brass tubes, each 30 cms. long and *19 cm. external radius. 

 These are inserted in a steam-trap (fig. 2) so that steam 



Fig. 2. 



issues simultaneously from both. One of the tubes is coated 

 in succession with increasing thicknesses o£ asbestos paper, 

 while the other has throughout a single coating of asbestos 

 paper, the object of this being to make the character of the 

 surface as nearly as possible the same for both, and thus 

 promote an equality in the values of the emissivity for them. 

 Under the orifice of each tube is placed a collecting test- 

 tube into which drops the steam which is condensed in the 

 pipes. The amount collected from the more thickly coated 

 pipe varies with the thickness of the coating. By taking the 

 ratio of this amount to that collected from the thinly coated 

 one, a value is obtained which may reasonably be taken as 

 being more independent of varied conditions of the flow of 

 the steam than if the value for a thin coating were deter- 

 mined once for all. The two tubes when coated each of them 

 with a single layer did not correspond to equal condensation 

 owing to some small difference of circumstance. The ratios 

 obtained have subsequently been raised 5 per cent., so as to 



