Prof. A. A. GRUSHOV.

THEODOLITE FOR SURVEYING IN MINES AND ON THE SURFACE

At the present time our geodetic factories produce small theodolites exclusively with a central telescope. This is explained chiefly by the fact that we require a larger number of theodolites for surface surveys, and for the latter a theodolite with a central telescope is the most convenient, since when measuring angles with an instrument having an eccentric telescope it is necessary to introduce the corresponding correction for “eccentricity.” Although the indicated error can also be eliminated mechanically, for this, as is known, the measurement must be made in two positions of the telescope and the arithmetic mean taken from the results obtained.

Therefore, where we are dealing with the need to measure angles in two positions of the telescope, the theodolite with a central telescope perhaps has no special advantages; but where it is possible to limit oneself to measuring an angle with one position of the telescope, the inconvenience of a theodolite with an eccentric telescope becomes quite appreciable. From this point of view, the theodolite with a central telescope is the most convenient for surface surveying, in which measurement of an angle with one position of the telescope is often used.

But in underground work, when, in order to connect mine surveying with surface surveying or to connect the surveys of different levels of a mine with one another, it is necessary to run traverses in steeply inclined and even vertical workings, a theodolite with a central telescope puts the mine surveyor literally in a hopeless position. In these cases work can be carried out only with a theodolite with an eccentric telescope. The eccentric telescope has an indisputable advantage over the central one also in carrying out astronomical observations, since with existing small theodolites with a central telescope, observation of the Pole Star already becomes difficult in our latitudes; observation of stars with an even smaller zenith distance becomes completely impossible.

Taking all this into account, and especially the nature of the mine surveyor’s work, who has to carry out both underground and surface surveys, and sometimes astronomical observations as well, the author set himself the goal of assembling a theodolite in which one and the same telescope could occupy both the central and the eccentric position. At the same time, for reasons of economy, the author followed the line of least

resistance and outlined a redesign of the theodolites already being produced by the Geofizika factory.

At the present time, such a redesign of a theodolite from the Geofizika factory has also been carried out by the author, with the participation of V. P. Zhitkovsky, mechanic of the geodetic laboratory of the Tomsk Industrial Institute.

In addition, certain additions and changes have been introduced into the rebuilt theodolite, which will be discussed below.

The general appearance of the rebuilt theodolite with the telescope set in the eccentric position is shown in Figure 1-m, and Figure 2 shows the appearance of the same theodolite with the telescope set in the central position.

Fig. 1.

As can be seen, the essential matter comes down to the following: onto one end of the horizontal axis of the theodolites produced by the Geofizika factory there is built up a cylinder \(Z\), which is a continuation of the horizontal axis of rotation of the telescope. The build-up is carried out by screwing it onto the screw thread already present at the end of the horizontal axis; for this purpose, in the body of the cylinder being built up, along the direction of its geometrical axis, a corresponding thread is made.

At the other end of this cylinder, whose length permits circular sighting with the telescope in a vertical plane, there is made, in one piece with it, a cylindrical sleeve whose axis is perpendicular to the horizontal axis of rotation of the telescope. The radius of the inner surface and the length of the sleeve, which serves as a socket for setting the telescope in the eccentric position, are respectively equal to the diameter and length of the cylindrical socket already present on the horizontal axis of rotation of the telescope, in which the telescope is set in the central position. When the cylinder built up onto the horizontal axis of rotation of the telescope is screwed in as far as it will go, the axes of both sockets (for the central and eccentric positions of the telescope) are parallel. Some deviation from parallelism will not be of special significance, and it will only be necessary, for the eccentric ...

positions, separately to determine the “zero position” and the collimation error. Incidentally, the cylinder that we have added to one end of the horizontal axis does not produce any noticeable loading, but it eliminates the imbalance present in the theodolites manufactured by the plant: the half to which the vertical-circle axis is attached proves heavier than the other half by approximately the weight of the added part.

Thus, for making astronomical observations, or for carrying out surveying work in steeply dipping or vertical workings, the telescope can be transferred and installed in the socket arranged at the end of the added cylinder.

Fig. 2.

Here it is not superfluous to point out the change in the fastening of the telescope in its socket which we introduced and recommend that the “Geofizika” plant introduce.

At present the “Geofizika” plant fastens the telescope in its cylindrical socket, located on the horizontal axis, by means of two diametrically opposite screws entering the body of the eyepiece elbow. This fastening at only two points—if allowance is made for an insufficiently well executed fit (there is a clearance of about 0.2 mm between the surface of the telescope and the surface of its socket), and for some damage to the threads of the fastening screws—can allow the telescope to wobble.

Our fastening of the telescope in its socket consists of the following: first, in the socket made by us for the eccentric position of the telescope, the fitting is performed more carefully, owing to which the telescope—even without fastening—has no noticeable wobble. Secondly, on the eyepiece elbow, at the place where it approaches the socket on the eyepiece side, we soldered on a ring (a ferrule) \(cd\), the edges of which are ground to a cone.

In Fig. 3 a section is shown by a plane passing through the axis of the telescope.

In it \(AB\) is the eyepiece elbow, \(a\) is the ring soldered onto the eyepiece elbow with conical edges; \(mnkl\) is the cylindrical socket in the co—

into which the tube enters, and \(CD\)—the objective elbow, by its thread screwing onto the corresponding screw thread at the end of the eyepiece elbow. When the objective elbow is screwed on, the edges

Fig. 3.

of the latter bear against the wall \(kl\) of the sleeve \(mnkl\), and the eyepiece elbow \(AB\) is “drawn” into the socket until the conical surface \(cd\) bears against the wall \(mn\). Thus, the objective elbow itself serves as the fastening device. Such a connection is undoubtedly the simplest and the strongest. Moreover, this connection makes it possible, by slightly unscrewing the objective elbow, easily and quickly to turn the reticle through any angle; the author used this circumstance and proposes using it to shift the stadia reticle threads from the horizontal to the vertical position, so as to be able, in order to avoid error from differential vertical refraction, to use the same stadia reticle also for determining distances on a horizontally set staff. This same rotation of the eyepiece elbow eliminates the necessity of bringing the reticle threads into the horizontal or vertical position by rotating the reticle itself, which was unavoidable with the fastening used by the so-called “Geofizika,” and which, as everyone knows, considerably complicates the work of eliminating collimation error, since, when the reticle’s adjusting screws are loosened in order to rotate it, the position of the line of sight may be disturbed.

To set the threads in the horizontal and vertical positions, we attached a projection \(y\) to the ring \(cd\) (see Fig. 1), and in the wall \(mn\) of the sleeve we installed two small rods \(x\) (at a distance of \(90^\circ\) from one another), which pass through the adjusting screw. In other words, we used the same device that is usually employed in levels for bringing the threads into the horizontal position. When projection \(y\) bears against the end of one adjusting screw, the threads of the stadia reticle occupy the horizontal position, and when it bears against the end of the other screw—the vertical position.

Finally, for the purposes of centering the instrument in mine-surveying observations, in the middle of the horizontal axis of the remounted

we have installed a centering device \(t\) (see Figs. 1 and 2), a vertical section of which is shown in Drawing 4.

At the bottom of the cylindrical box \(MN\), which has a screw thread on its outer side, there is placed a cylindrical center \(r\), ending above in a cone and capable of moving along the bottom of the box \(MN\).

A cover \(RS\) is screwed onto the top of the box \(MN\); when tightened, it presses upon the upper flat edges of the center \(r\) and fixes it in the position established after adjustment.

Fig. 4.

In the case where the telescope is set in an eccentric position, a cylindrical counterweight \(P\) (22 mm high), whose diameter is approx. 65 mm, is screwed onto the other end of the horizontal axis (see Figs. 1 and 2). On the side facing the vertical circle, in order to leave unchanged the position of the level attached to the alidade of the vertical circle, the counterweight is ground down (see \(Q\)). Both the counterweight \(P\) and the added portion of the horizontal axis can be removed and placed either separately or in special recesses, which may be made in the same case in which the theodolite is packed.

To avoid contamination, the free socket is closed in our instrument by a hollow copper plug made for this purpose.

Concluding the description of the theodolite we have remounted, we consider it necessary to express the following wishes:

1. That the Geodetic Factory should mark divisions on the vertical circle along its entire circumference, and not only within \(60^\circ\) from the zero diameter, as is done at present. At the present time divisions on the vertical circle are applied only up to \(60^\circ\) because the central telescope of the theodolites being produced permits measurement of angles of inclination only up to this limit.

2. For the purpose of obtaining the strongest possible connection of the telescope with the horizontal axis and avoiding bending of the latter, it would be better to connect the horizontal axis rigidly with the added portion, and to increase the diameter of the journals by which the axis rests in the bearings, as compared with the existing size, by approximately 3 mm. The fact that a socket rigidly connected with the horizontal axis (for the eccentric position of the telescope) will make it impossible to put the clamp of the micrometer apparatus on the telescope axis (see \(L\) in Figs. 1 and 2) will not be an obstacle, since the clamp can be made of two parts (the upper \(a\) and the lower \(b\)—see Drawing 5), which, after being placed at the corresponding point of the horizontal axis, are then fastened together with screws \(c\).

Fig. 5.

The added portion of the horizontal axis

It may also be made removable, but the diameter of the journals of the horizontal axis should definitely be increased; moreover, in this case, in order to avoid weakening the connection of the built-up part with the main body of the horizontal axis, we recommend that, when working with the off-center telescope, the latter be set in the vertical plane by rotating the counterweight. This will not in the least complicate the work, since, by holding the instrument by the counterweight, we are able to set the telescope both in the vertical and in the horizontal plane; owing to the large diameter of the counterweight, sighting the target point in the vertical plane will be carried out more smoothly and accurately.

As for the counterweight, it should, of course, be made removable and attached only when working with the off-center telescope.

Thus, as is clear from the description, the redesign of the theodolite of the “Geofizika” plant proposed by the author will entail a comparatively small increase in manufacturing cost for the plant, but the field of application of such an instrument will be considerably expanded, which in the final result will provide great convenience and economy.

Theodolite by Prof. A. Trustschoff

The distinctive feature of my theodolite consists in the fact that its telescope can be mounted in both a central and an eccentric position. For this purpose, on the horizontal axis—on the side opposite the vertical circle—there is fastened a cylinder forming a continuation of the horizontal axis; at its free end there is a socket into which the telescope is shifted from the center into an eccentric position; at the same time, a counterweight is screwed onto the other end of the horizontal axis.

The geometric axis of the eccentric socket is parallel to the central socket. A slight non-parallelism is of no essential importance, since instrumental errors are eliminated when an angle is observed in the two telescope positions.

It follows from this that the theodolite described above can be used both for mine surveying and for ordinary field-surveying work.

It remains for us to note that, in our theodolite, the fastening of the telescope in its socket has been modified; compared with the fastening used by the firm “Geophysika,” this permits the telescope to be set into one of the sockets more quickly and more securely, and thereby allows the stadia hairs to be transferred easily and rapidly from a horizontal to a vertical position, consequently making it possible to use the same stadia hairs for reading the distance on the measuring plate set up vertically and horizontally.