EARL ROSSE’S OBSERVATIONS ON THE NEBULiE. 
501 
divided. Each system consists of one triangle with its point of support directly under 
its centre of gravity, upon which it freely oscillates. This triangle carries at its angles 
three similar points of support for three other triangles, under their centres of gra- 
vity, and they again at their angles carry in a similar way cast-iron platforms formed 
of thin ribs so as to make a kind of irregular open-work grating, supported under 
their centres of gravity. These platforms are all of equal area though not of similar 
shape. As there are three systems there are therefore twenty-seven platforms, which 
together make a circular disc about an inch in diameter less than the speculum : 
when arranged however a little apart so as not to touch, they make a disc about the 
same diameter as the speculum. Each platform is coated with greased cloth, and 
may be considered as bearing up one twenty-seventh of the vveight of the speculum. 
Bet',veen the platforms and the speculum pieces of tin plate are inserted to diminish 
the friction as much as possible. The platforms being of open-work, they do not pre- 
vent the water in which the speculum is immersed from freely carrying away the 
heat as it is developed during the process of polishing, which is essential. 
It is evident that a speculum so supported will be practically free from strain while 
in a horizontal position, provided the due action of the levers is not interfered with 
by any disturbing force ; it will be very much in the same condition as if floating in 
a vessel of mercury ; when it ceases to be horizontal however new forces come into 
play : part of the weight must then be resisted by pressure against the edge. Four 
very strong segments of cast iron, each about one-eighth of the circumference, were 
adjusted to the edge by screws, the segments bearing upon the massive castings 
which sustained the three primary supports of the lever apparatus. Provision was 
made to allow a little motion perpendicular to the plane of the speculum, to guard as 
much as possible against strain from the elasticity of the lever apparatus, which was 
however very small, the yielding being less than one-fortieth of an inch. 
The two specula of 3 feet aperture I have so long employed are mounted on a 
similar principle : they have however fewer points of support, and by a little sacrifice 
of the condition of perfect equilibrium, the whole system of levers was thrown with- 
out difficulty almost exactly into one plane. They are free from perceptible flexure 
in the different positions of the instrument. With the two specula of 6 feet diameter 
the case was otherwise. The 3-feet specula, weighing each about thirteen hundred 
weight, were very much stiffer, in proportion to their weight, than the 6-feet specula. 
To have made the 6-feet specula of equal proportionate stiffness, either they should 
have been enormously heavy, or the material should have been so disposed as to give 
greater stiffness than when simply cast into the shape of a solid disc. Some years 
ago it was ascertained by experiments, but on a small scale, that it would be prac- 
ticable to dispose of three-fourths of the material of a speculum so as to secure a 
great increase of stiffness ; the form adopted was a system of hexagonal cells. Whe- 
ther on a great scale the difficulties would be too serious to be surmounted is a ques- 
tion ; however it is with solid discs we have had to deal. The relative stiffness of 
