64 Mr Tait on a Portable Diorama. 



With regard to the elastic force of the steam required for 

 the distributor, I may here state that I have used it at pres- 

 sures varying from 3 lb. to 35 lb., and invariably with the 

 same effect as to the prevention of smoke. How far the dif- 

 ference in the elastic force of the steam projected into the 

 furnace affects the amount of economy of fuel I cannot say, 

 as all the trials made with the view of ascertaining this were 

 with high-pressure steam. 



It has been supposed by some, that however beneficial this 

 process may be as to the prevention of smoke and economy of 

 fuel, it is liable to serious objections from the destruction of 

 the fire-bars, owing to the want of air passing up between 

 them, and also from the injury occasioned to the boiler by the 

 action of the steam from the distributor. As to the former, 

 suppose it were to happen, it would be more than counter- 

 balanced by the saving of fuel ; but, so far as I have seen, the 

 bars are not more acted on than in other cases; indeed, 

 I would say less so ; for, as there is very little or no air pass- 

 ing up from the ash-pit, the heat evolved at the bars is by no 

 means so great there as it is when air is rushing up through 

 the fuel. As to the boiler, the only one I have minutely ex- 

 amined is that with which I conducted the trials, the results 

 of which I have given. After being in use for about eighteen 

 months, during which I was from time to time experimenting 

 with it, I had it examined by experienced engineers, who all 

 certified that it had not sustained the slightest injury. 



On producing the effect of Fog in a Portable Diorama^ con- 

 structed by George Tait, Esq., Advocate. Communicated 

 by the Royal Scottish Society of Arts.* 



In a description of a portable diorama which I exhibited to the 

 Royal Scottish Society of Arts on 22d November last, I stat- 

 ed that the effect of fog may be represented by painting the 

 objects intended to be affected by it, on a second surface be- 

 hind the first, by admitting light behind, and by gradually 

 removing the surfaces from each other, or bringing them into 

 contact. 



* Read before the R4>yal Scottish Society of Arts Uth April 1842. 



