Reflective Goniometer. 75 



wrought Iron heads. I have lately seen, at the Philadelphia Water 

 Works, the range of boilers, constructed several years ago, on the 

 above principle, by Oliver Evans himself, removed, on account of 

 their use having been superseded by water power. Although these 

 boilers had been for several years employed under a pressure of 100 

 and 150 lbs. per square Inch, yet the heads did not appear to have 

 suffered in the least degree from exposure to this force. Hence the 

 French instructions, forbidding the use of plain cast iron heads for 

 pressures above 1 J atmospheres, do not seem to be founded on suffi- 

 cient experience of their actual value. 



Art. XIII.- — A simplification of Dr. Wolla"4on''s Reflective Goni- 

 ometer ; by R. Graves, Jr., Civil Engineer. 



Having found much difficulty attendant upon the use of Dr. 

 Wollaston's goniometer, I was led to the construction of the one, a 

 description of which follows. 



I am aware that it has been said, " that the value of a reflected 

 angle cannot be found by mere inspection,''^ yet it will readily be 

 granted that, let a person assume any degree of intenseness of re- 

 flection upon one plane of the angle sought, and let him detect the 

 same degree of vividness upon the remaining plane, he will have 

 inspected the angle formed by the meeting of these two planes, the 

 which, if done upon any graduated surface whose revolving can be 

 determined, will give the value of the angle sought. Now the 

 only difficulty lies in detecting ejMaZ degrees of intfenseness. This 

 I have tested both by the amount of a double right angle, and by 

 affixing an angled plate upon the revolving plane. It can be done 

 by simply catching the darkenings on the edges, as they approach the 

 axis of the planes the value of whose angles is sought. There 

 are two kinds that have been used. One made of brass with bands 

 and braces instead of being solid, is an expensive though light and 

 pretty instrument. The other is made of tvell seasoned wood ; It Is 

 less neat in appearance, but is simple, cheap and can be made by al- 

 most any person, (I made the one I use myself.) It Is the latter 

 Instrument that I shall describe. 



Elevation view. — The dark concentric ring AA, bounding II, is 

 made of mahogany one half or three fourths of an inch in breadth 

 upon the anterior surface, two inches deep till it meets II, it is then 



