180 REPORT—1850. 
On a Folding Dome for Observatories. By W.S. Jacos, C.E., HELC. 
Astronomer at Madras. Communicated by Prof. P1azz1 SMyTH. 
Revolving domes for equatorial instruments have been greatly improved and sim- 
plified of late, but are still expensive ; they require skilled mechanics to execute them, 
and are not easily made portable : the folding dome, however, possesses these proper- 
ties. It consists merely of eight triangular shutters, hinging on an octagonal wall 
plate, and closing against each other so as to form an octagonal pyramid. 
No fixed frame- work is required in this dome, as the shutters support each other ; 
two even will stand together, so that it is possible to open any number of sides of 
this octagonal pyramid from one to six: they open outwards, and are supported 
then in some simple way, as by a bamboo prop. 
One of these domes, 8 feet in diameter, was made by Mr. Jacob seven years ago ; 
it was all constructed by an ordinary village carpenter in India: the shutters were 
triangular frames covered with canvas ; and the sides being bevelled, no inconvenience 
was found from leakage during rain. The dome has twice been taken down and 
carried across the country above 300 miles in a bullock cart, over bad roads or no 
roads at all, and is now again re-erected and in use at Bombay. Its first cost was 
only £25. 
On a Method of Supporting a large Speculum, free from sensible Flexure, in 
all Positions. By Wii.1aM LassELt, F.R.S.L. ὃ £., F.R.A.S., §e. 
Before describing the method I propose of overcoming the tendency to flexure in 
large specula, I will briefly describe the evil which I desire to remove. 
Your President has aptly described the nature of the metal with which we have to 
deal, as “‘ harder than steel and more brittle than glass.”” Yet with these qualities 
one would scarcely expect that the bending of a large speculum would be one of the 
greatest difficulties in its use. No one unacquainted with the subject would be pre- 
pared to believe that a plate of this metal, two feet in diameter, and two and a half 
inches thick, would bend by its own weight, if supported only by its circumference. 
It is however the fact, that if such a speculum be ground and polished with the ut- 
most accuracy to the requisite curve, it will receive such an amount of distortion by 
being supported only at its circumference, or on three points placed at its back, as to 
be greatly impaired in its performance, if not rendered totally useless as a telescope. 
A very much smaller speculum will be materially injured in its figure by unequal 
support; but I shall chiefly refer to a two-foot speculum in what I have to say, as to 
a mirror of that size my remedy is intended to be applied. 
The speculum of my twenty-foot telescope is at present supported by eighteen 
points, or rather small discs of about 13 inch diameter, which are placed at the ends 
of a system of levers so distributed and compensated as to bear, severally, one-. 
eighteenth part of the weight of the speculum. I will endeavour briefly to explain 
how this is done. 
Suppose the speculum divided into three equal sectors of 120°, one of which is re- 
presented in fig. 1. Let a concentric arc H I be drawn of such radius that the area 
of its sector shall be equal to one-third the area of the larger sector. An additional 
line D E, bisecting the arc A B, and drawn in the direction of a radius, will divide the 
larger sector into three portions of equal area. These three portions may be again 
bisected by the supplemental Jines FG, EC and KL. The sector is now divided 
into six portions of equal area; and therefore, similarly, the entire speculum would 
be divided into eighteen equal portions. Let the centres of gravity of these six por- 
tions be indicated by the centres of the six small circles. These circles represent 
discs of brass in immediate contact with the back of the speculum. They are divided 
into three pairs, the individuals of each pair being connected by a bar or lever, at each 
end of which there is a hole, loosely fitted by a pin projecting downwards from the 
centre of the brass disc above it. The under surfaces of the discs are small segments 
of spheres, allowing a slight rocking motion of the disc for perfect adjustment to the 
back of the speculum ; the centres of these levers are again supported by the ends 
of a triangular plate or lever M N O, in the same manner as the discs are supported 
by the first three levers. Finally, this triangular lever is supported at its central - 
point P by an ultimate screw or stud fixed in the end-plate of the tube. The trian- 
