Polytechnic Association. 785 



Mr. Fisher — The evaporation per foot of surface seems to have 

 been quite small. 



Professor Thurston — It is the evaporation with a natural draft, 

 with low chimneys. In the locomotive you have a forced draft, and 

 in the Cornish boiler you have high chimneys, so that their evapora- 

 tion will be greater. 



Mr. Fisher — It appears to me that these boilers did not perform 

 so efficiently as locomotive boilers. 



Professor Thurston — You cannot judge of the evaporation of a 

 boiler that has not been tested in this way. A gentleman told me 

 that on testing his boiler he found that it evaporated eighteen pounds 

 of M r ater to the pound of coal. I told him that was twenty per cent 

 more than a perfect boiler would evaporate ; so that it was evident 

 that a large amount of water had passed off with the steam. He 

 said that he had held his handkerchief in the steam and it came out 

 dry. He went back and made another trial, and made it fifteen 

 pounds ; but on trying the thermometer he found that it indicated a 

 higher temperature than that due to the pressure. Another experi- 

 ment brought down the apparent evaporation to twelve pounds and 

 a fraction. I have no doubt that in his first experiment he had made 

 the performance at least double what it really was. 



Dr. Van der Weyde repeated his suggestion, given in his lecture 

 on heat, that an excellent test is to blow steam into water until we 

 have added a pound to the weight of the water. This requires no 

 apparatus, and enables us to determine, from the rise of the temper- 

 ature of the water, how much water was discharged mingled with 

 the steam. 



Professor Thurston — That would be a very accurate and conve- 

 nient method for ordinary purposes. 



Mr. W. E. Partridge — A further application of this method has 

 been made use of by several engineers in this city, in testing steam 

 pumps. Thermometers are placed in the water that enters the pump, 

 and in the water as it passes away. The radiation is very little, and 

 the results compare favorably with those taken by more expensive 

 methods. 



Mr. Geddes — Would the tests made by this committee enable 

 them to advise the Board of Managers as to the form of boilers to be 

 purchased for the fair ? Whether it should be one of the forms 

 reported upon or some other form of boiler ? 



Professor Thurston — All our examinations have been with, regard 



[Inst.] 50 



