On Tuning Musical Instruments. 185 
to trace upon the horizontal plane is determined. This el- 
lipsis being traced, we move the mirror in the same way 
as in fig. 9, by means of a machine which it would be 
€asy to conirive, ‘Hence I conclude, that by following the 
~ Same principles as before we may burn an object placed at 
a great distance. In this manner therefore Archimedes 
may have burnt the fleet of Marcellus. 
It will be easy to perceive that the mirror EF (fig. 8) 
and DK (fig. 9) might have an oblique position on the 
plane of the equator, provided in the two cases it was fixed 
with the straight line AB perpendicular on the plane of the 
equator. 
I shall conclude this essay with two observations. If 
the mirror DK, instead of having a fixed position, was 
moveable in the iron band CDE (fig. 9), and if this mirror 
was adjusted in order to carry the images to R the mid- 
dle of CE, it is evident that 1f we made this mirror have 
its axis YZ constantly directed to the sun’s centre, the foe 
eus would remain at the point R during the whole day and 
every day of the year. 
{ ca! the axis of aburning mirror the axis of the co- 
noid, one part of the surface of which forms the burning 
mirror. 
Upon the same principles it would be easy to make a 
mirror of retraction, so as to keep its focus constantly at 
the same point. 
et AG (fiz. 10) be an iron rod parallel to the axis of 
the earth ; let CDE be an iron band folded into the are of 
acircle, having for its centre the point M taken.on the 
axis of the rod AB; let KL be a glass moveable round an 
axis perpendicular on the plane which passes by AB and by 
the middle of the band CDE. Let us suppose that by 
means of a screw, we keep, during the whole time that the 
sun is upon the horizon, the glass parallel to the sun; itis 
evident that the focus Q will remain fixed at the same 
point of a hollow RDS placed on the band CDE. 
XXXII. On Tuning Musical Insiruments. By a Cor- 
RESPONDENT. | 
To Mr. Tilloch. ° 
Sir, W irnovr entering at all into the disputes about 
words, the characters of authors; what silly tuners do, or 
say; and whether /ifths should be this or that way tem- 
; pered, 
