ELASTIC FORCE AND EMISSION OF STEAM. 249 



mine the place of the eye flop, but a focus, where the Image 



of the fmall fpeculum is formed, and this will be more or lefs 



accordingly as the diftance between the lens and the fpeculum 



is lefs or more. For inftance, take a lens of 1,5 inch focus, 



which would fuit a fpeculum of 9 inches focus, and the dif- 



tance of the fmall fpeculum will be about 14. inches from the 



eye glafs. Then 14xl,5 



'= 1,68 will be the true diftance 



of the eye flop, or in other words, the focus of rays proceed- 

 ing from an object placed 14 inches beyond the lens, almoft 

 two inches farther than the folar focus. 

 1 am. 



Dear Sir, 



Your mofl obedient Servani, 

 J. C. HORNBLOWER. 



IX. 



Account of a new Apparatus conjirudedfor the Purpofe of mea^ 

 furiiig the FJaftic Force, and regulating the Emijion of Stca?n 

 from the Boiler in which it is generated. Communicated hy the 

 Inventor, Mr. Arthur Woolf, Engineer. 



A LATE XIV. exhibits a meafured fe6lion of the felf-a6tlng Self a^ing and 

 and regulating fleam valve. A A reprefents the upper part of ""^sulating fleam 

 the boiler, having its mouth or neck cylindrical, and clofed by 

 a well-fitted, but eafily moving valve plug B B C C, which is 

 in fad a metallic tube, open at bottom and clofed above, by a 

 cap-piece B B, that by its chamfered rim or projecting part 

 affords the accurate valve-clofure when down. The interior 

 parallel lines at D fliew the place where a long pertoration is 

 made through the fide of the cylindrical part of the valve plug 

 from its cap, nearly down to the bottom ; which perforation 

 affords a paffage for the fleam, increafing in magnitude as the 

 elaflic force caufes the valve to rife. E is the fide paffage for 

 conveying the fleam to its place of operation. F is the rod or 

 tail of the valve pafling through a fluffing box above, and at- 

 tached by a chain to the fedor G, and by its means moving 

 the lever that carries the ball H. 



The 



