Reflective Goniometer. 15 
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their use having been superseded by water power.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 e 14 atmospheres, do not seem to be founded on sufli- 
cient experience of their actual value. , 
Art. XIII.—A simplification of- Dr. Wollaston’s Reflective Goni- 
— ometer ; by R. Graves, Jr., Civil Engineer. 
. Havre 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- 
ction 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 equal degrees of intenseness. This 
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 
y 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 well 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 [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 
