35 2 Dr, Herschel*s 'Description of a 
The firmness of the foundation in the direction from south 
to north being thus secured, it became equally necessary to 
provide for strength in the support from east to west. For 
this purpose there are three latitudinal cross beams, H H, II, 
K K, bolted down upon the former longitudinal ones. That 
which crosses the centre is 45 feet 2 inches long, and 12 
inches broad ; its thickness, like that of all the ground timber, 
being 6 inches. The other two are about 39 feet 9 inches 
long, and 6 inches broad. They project beyond the circular 
foundation wall about 8 inches, while the middle one projects 
12. The use of these cross beams is to receive six supporters 
upon their respective ends, at the places which are marked 
with an ellipsis ; the supporting beams which stand upon 
them being round and inclined towards the ladders, which 
they are to keep steady in the east and west direction. 
Under each end of the principal beams at A A, B B, C C, 
H H, II, K K, is placed a roller which rests upon the outer 
foundation wall. The three latter of these beams being placed 
higher than the former, have a piece of a proper thickness 
under the ends to bring the bottom of them to the level with 
the former. The rollers are set in iron frames, and bolted to 
the beams, so as to be directed to the centre of their motion. 
They are 8 inches long, and 6 in diameter. The construction 
of those which are under beams that come from the centre, is 
expressed in fig. 4. ; but the irons which hold the rollers un- 
der beams in other directions are more or less eccentric, as 
may be seen in fig. 5. 
No other fastening in the whole machinery of the wood- 
work has been admitted but screw bolts ; as tenons of any 
kind, in an apparatus continually tb be exposed to the open 
