MATHEMATICAL AND SURVEYING INSTRUMENTS. 69 
the box from the canal. OK is an entirely inclosed space, cut off by the 
partition P. The cubes, 8 and T, fill the boxes C and D, so as to move 
freely up and down in them without shaking. These cubes carry above 
the diopters U and V, shown in fig. 44, having an aperture divided by a 
cross hair, X. Figure 43 is an under view of the instrument. The left 
hand side of fig. 42 exhibits it in section, and the right hand side in a lateral 
view. ‘The entire instrument is connected by means of the strip ab (fig. 44), 
and the screw c, with the socket d, with which it is placed upon a stand: it can 
be fastened by the screw e (fig 42). Touse the instrument, the canal EF is 
filled with mercury, whose surface naturally assumes a horizontal position. 
The two equally heavy, and in all respects similar cubes, 8 and T, are placed 
upon the mercury, which has flowed into the boxes C and D, and the cross 
hairs of the diopters will be in a horizontal plane, which can be produced 
by the sighting of signals. When not in use, the bottom G is pushed from 
under the bottom H, and the instrument is so constructed, that the mercury 
enters into the spaces M and N, which then are closed by sliding back the 
bottom G. The whole may then be transported without shaking the mer- 
cury. On account of the influence of the mercury upon brass and copper, 
these metals must be avoided. The cubes, with the diopters, should be 
constructed principally of ivory. 
All these forms of levels being constructed with diopters or sight vanes, 
are applicable only to short distances ; for long distances telescopes must 
be employed. The tube level is employed in this case for securing a 
horizontal position of the axis of the telescope. The tube level is preferable 
to the tub level, on account of its greater length and sensibility, and to the 
mercurial and water levels, for its greater convenience in use, as also for its 
superior delicacy. | 
Yg.46 represents the simplest form of levelling telescope. This is placed 
on the plate of the plane table, by means of the tripod A, and the level 
rendered horizontal by means of the setting screws, a, a,a. The foot A is 
hollow, and in it turns a pin with a plate, B, upon which rests CD, the 
carrier of the level and telescope. The level, EF, rests in a peculiar frame 
upon this carrier, one end on a spring which is compressed by the pressure 
screw G. This spring serves to correct the level in case its axis should not 
be parallel to that of the telescope. In the supports erected at C and D, 
the telescope KL rests ; it is held there by the clip springs H and I, which 
can easily be thrown back, to allow the telescope to be taken out and 
reversed. In the focus of the ocular there is a cross hair of human hair or 
spider’s web, which is so fastened in a frame as to be capable of being 
placed exactly in the axis of the telescope, by means of the screws 6,b. In 
this instrument the axis of the telescope answers to the sight line of the 
diopters in the water level. When it is once set up horizontally, levels 
may be taken in every direction, by turning the level and telescope bearer 
on the foot A. 
The level and compass (pl. 5, fig. 45) is a more complicated instrument, 
possessing the advantage of being able to measure the angle formed by one 
leg levelled with another. This level has its own stand, with three feet, P, 
69 
