Stonzy—-Oompressed Air Support for Specula. 269 
A, B, OC, of which A and C communicate with one another, and 
contain compressed air, while B is open to the atmosphere and 
contains a regulator which causes 
the pressure within A and C to 
vary by the right law when the 
telescope is moved from one alti- 
tude to another. This regulator 
consists of m, a corrugated disk 
like the cover of the vacuum-cham- « 
ber of an aneroid barometer, and 
na weight which leans against m, 
and is kept from slipping sideways 
by wires p, g, 7, of which p and q 
are shown in the figure. If the 
pressure within the cells C and A (which communicate with one 
another) becomes too great for the proper support of the mirror, n 
will move forwards and cause the index / to make electric contact 
with one of two studs, in consequence of which a valve is opened 
that lets some of the imprisoned airescape. If, on the other hand, 
the pressure is too small, » moves backwards, contact is made 
with the other stud, and a passage is opened between a gasometer 
holding compressed air and one of the chambers A and C. This 
regulator will vary the pressure within the chambers A and C 
when the telescope is moved about, and will cause it, in each posi- 
tion of the telescope, to settle down to that which is precisely the 
right amount to support the mirror without flexure, at that elevation. 
The mirror may be brought into a pre-determined position by 
being placed just in contact with studs x, y, s, and then cemented 
round the edge by some cement, like Archangel pitch, which will 
adapt itself to the mirror and hold it firmly without straining it. 
If necessary, the front of the cell may be strengthened by flanges 
to prevent distortion when the cell is placed on its side. 
Mounted in such a cell, which can be rigidly fixed within the 
tube of the telescope, it is anticipated that the line of collimation 
of the instrument will be as fixed, and may be as much depended 
upon as that of a refractor; and if this be so, reflecting telescopes 
may be made available for photographing the heavens and photo- 
graphing the spectra of stars, in the hands of any astronomer 
engaged in this class of work. 
