OPT 
mounted in this manner, till one end of the tube vyas 
prefled againft fomething that was very Heady and unelaf- 
tic. It is quite aftonifliing what a change this produces. 
“ We took a very tine teiefcope made by James Short, and 
laid the tube on a great lump of foft clay, prefling it 
firmly down into it. Several perfons, ignorant of our pur- 
pofe, looked through it, and read a table of logarithms at the 
diftance of 310 yards. We then put the teiefcope on its 
ftand, and pointed it at the fame object j none of the com¬ 
pany could read at a greater diftance than 235 yards, al¬ 
though they could perceive no tremor. They thought the 
viflon as (harp as before; but the incontrovertible proof 
of the contrary was, that they could not read at iuch a 
diftance.” 
When the length of the tube is three feet and upwards, 
and proportionably wide, fig. 9 reprefents a ftand that is 
greatly to be preferred to that reprelented by fig. 8. We 
lhall put the fame letters of reference to the fame parts, 
though there is a difference in the conftruCHons that may 
require explanation. This ftand was contrived and firlfc 
made by Tuiley, who, we underftand, claims alfo the in¬ 
vention of the three-armed brace F, in fig. 8, above de- 
fcribed. The contriver has evidently contemplated all 
the requifites for a good conftruflion, and has l'ucceeded 
in the execution of his plan : AB, as before, is the main 
tube; B, one of the eye-pieces; C, the adjufting fcrew 
for diftinft viflon, hid in our drawing behind the tube; 
but, in place of it, is feen the finder, attached above 
the eye-end of the main tube. (The finder is a finall 
teiefcope, attached to, or over, the main tube, the 
ufe of which is to bring the objeft readily into the field 
of view of the large teiefcope, which is not an eafy 
matter when the power is great; for, as the field of 
view increafes with a diminution of the power of the 
teiefcope, and vice verfd, the finall teiefcope faves much 
time in fearching for any objeCt that is vifible in it.) At 
D is a fliding-piece of metal with a cylindrical hole, 
through which the round rod HD pafies, and to which it 
may be fixed, by the prefling-fcrew D, in any given eleva¬ 
tion ; to this fliding-piece D, two rods DK, DK, are at¬ 
tached by two joints, and two other joints attach them 
below to the frame EK, fo that thefe rods K, K, are at 
liberty to rife and fall as the tube is elevated or deprefled, 
but not until the fliding-piece at D has moved along the 
rod HD. When the piece D is fixed to the rod by the 
fcrew of p refill re, it forms a point of bearing for the tube 
at a diftance from the centre of motion, which is at the 
centre of the tube’s gravity above the frame of brafs-work, 
feen in the figure; thus, the teiefcope is kept fteady by 
two points of bearing in every degree of elevation, though 
thefe points will recede from each other gradually as the 
teiefcope is deprefled towards an horizontal pofition. 
When the fcrew at D is turned back, the motion is fuffi- 
ciently quick; but, when it is faff, the flow motion is 
produced by the handle at H ; for, while this handle 
turns the rod, a fcrew cut in its interior end works in a 
fixed cock, near H, that has a female fcrew within it, 
and draws the fliding-piece and rod together towards the 
eye, and thus elevates the tube, while the joints of the 
rods K allow' a correfpondingelevation in them; fo that, 
without undoing the fcrew at D, a flow motion up or 
down is produced by merely turning the handle H, which 
motion, being free from jerks, is very pleafant. Between 
the brafs frame, bearing the teiefcope and the large wood¬ 
en frame EF, are three circular plates, the uppermofl of 
which is attached to the brafs frame, or may be faid to 
form the bafis of it, and has an axis of fteel fall; in its 
centre; the fecond circular plate is racked at the con¬ 
cave edge all round; and has a circular hole in the centre, 
juft large enough to receive the fteel axis we have men¬ 
tioned. The third circular plate forms the top of the 
wooden tripod, and has alfo a hole in its centre, juft fuf- 
ficient to admit the fteel axis above defcribed ; but its 
diameter is fomewhat lefs than the diameter of the racked 
plate.next above it, fo that a rim, made faft to the racked 
ics. ro? 
plate, furrour.ds it, in the manner of a box-lid; but 
there is no other {'aliening of thefe three plates together 
than the prellure occafioned by the fuperincumbent 
weight of the teiefcope, and of its fubjacent frame EKK. 
The axis, or fcrew, of the handle I is made faft to the up- 
permoft plate of the faid frame, and takes hold of tne 
notches in the racked plate below it ; fo that, when the- 
teiefcope is turned round in azimuth by a quick motion, 
it takes the frame under it, and alfo the racked circle, 
round along with it, while the ftand or wooden four- 
footed frame EF ftands quiefcent; but, w’hen the quick 
motion is finifhed, the handle I is ftill in its.place at the 
eye-er.d of the tube, and turning it round will give the 
requifite flow motion ; for turning the handle, in con¬ 
nection with the racked plate, turns the fuperincumbent 
frame and teiefcope, without any motion being given to ■ 
the racked plate itfelf, which is now kept down to its 
place by Ample preflure of its load above. The ftand, or 
large wooden frame, is braced in all directions, as may 
beleen in thedrawing, and might be advantageoufly made • 
of caft-iron, as it is not contrived for the convenience, of 
portability. 
The belt ftand for the Newtonian teiefcope. is that which 
is reprefented by fig. 10. in which A is the elevated mouth 
of a feven-feet tube, and B the place of the large ipecil¬ 
ium, that reflects the rays of light back to the final 1 dia¬ 
gonal plane metal near C, which, by a fecond reflection, 
brings them to a focus at the eye-piece below C, as feen 
in the_ drawing. Above C is the finder, the upper end 
of which has a finall achromatic objeCl-glafs, and the 
lower end the eye-glafs. The upper end of the tube refts 
on a fupport D, that is capable of being raifed or lowered 
flowly by a pinion on the axis of the handle under D, 
while the lower end refts on the horizontal bar of the 
frame EF, that .isTufpended by a pulley over F ; the four 
pivots a, b, c, and d, of the laid frame, Hiding in the open 
grooves, feen near thofe letters, in the main frame, keep 
the finall frame in any g.’.en fituation, and allow a free 
motion, firft down the vertical, and then down the in¬ 
clined, pieces, that compofe the main frame, as iovv as to 
G and H ; and, when the lower end of the tube has been 
deprefled into this fituation, the tube may have an eleva¬ 
tion approaching towards the zenith : for not only is the 
upper end elevated by the handle at J for the quick, and 
at D for the flow, motions, but the lower one is deprefled 
by the handle at I, round which the cord is coiled, that 
goes round a fixed roller at K, and two others at L and 
M, before it embraces the pulley N, and is hooked to a 
pin at O, above the frame. The reft of the main frame 
is fo clearly exhibited in the figure, that no farther de- 
fcriptjon of it is neceflary. In fome of the inftruments 
of this conftru&ion, when the handle J is omitted, and 
a quicker motion in altitude is required, and alfo a great¬ 
er elevation than can be given limply by the handle atD, 
the fecond fquare ftera that carries the pinion of the han¬ 
dle is raifed by hand, and kept to its elevation by means 
of a fecond rack, which is fet at liberty by prelfing a but¬ 
ton at P, connected with the fpring-catch of the rack, 
when this fquared Item is lowered again ; all which mo¬ 
tions will be readily comprehended by any perfon tole¬ 
rably acquainted with the mechanifm of rack-work. The 
quick motion in azimuth is given by Hiding the lo.ver 
end of the tube gently along the bar on which it refts, 
or by moving the whole frame, which goes on caftors ; 
but the flow motion is produced by the fcrew at D. It 
is fcarcely neceflary to add, that the eye of the obferver 
is applied to the fide of the tube near its mouth, when 
thefinderhas pointed the tube properly toits objeCt. This, 
ftand was contrived by fir William Herfchel, whofe expe¬ 
rience in the ufe of various ftands directed him to prefer 
one that is not liable to propagate vibratory motion to 
the large fpeculum, and that has a point of lupport near 
the upper extremity of the tube. 
The high magnifying powers of Dr. Herfchel’s tele- 
fcopes, made all the ufual apparatus for their fupport ex¬ 
tremely. 
