37 2 Dr. Herschei/s Description of a 
of these bars they are fastened by screws, which pass through 
the upper bar, and are lodged in the lower one. The same 
screws pass on through a moderate plate of sheet iron which 
closes the back, and is held by nuts upon them within the 
tube. The eight returning bars, at ab eg hi k, extend only to 
about 6 inches along the tube ; but they are immediately re- 
ceived by other eight bars of the same size, which are screwed 
upon them. These bars are made tapering, so as only to mea- 
sure \\ inch broad, and thick at the ends ; and they are g 
feet 8 inches long. The middle bar is turned over about 16 
inches, and made tapering ; and the bar which meets it is laid 
under it, and also made tapering to answer the former. The 
pivot goes through both, and they form, as it were, only one 
bar ; this is soon reduced gradually, and at the end measures 
i-| inches by \ ; its length being the same as the rest. 
The segment eng is cut off to leave an opening, which is 2 
feet broad at the sides. A cover of the same shape, with the 
piece cut out of the tube, is laid upon the place, to overlap the 
opening properly. But this would not have been sufficient : 
for, after observations at night, this cover, though close enough 
to preserve the inside of the tube from damp or wet, would 
itself be covered with dew or condensed vapour. And by tak- 
ing it off in order to secure the mirror, many drops of water 
would unavoidably fall upon it from this wet cover. To pre- 
vent this, an outward cover has been applied, which com- 
pletely preserves the inner one from moisture. 
The tube being much too weak, in this place, for the sup- 
port of the mirror, a piece consisting of three sheets of iron, 
2 feet 4 inches broad, £ thick, and dove-tailed together so as 
to be long enough to reach from c around b am Ik eh to g, 
