Fig. 152.—5-inch transit theodolite (old form).
Larger image
367.—The size of a theodolite is fixed technically by the diameter of the line of division upon the horizontal circle. A 5-inch or 6-inch theodolite is the largest size that may be carried comfortably in a single case; and no great advantage is gained by having an instrument beyond this size if the work is that of the ordinary surveyor on town and county surveys. The verniers of 4- and 5-inch instruments read sharply to single minutes of arc, which is as nearly as can be plotted with any degree of certainty with an ordinary protractor reading by vernier also to minutes only; 6-inch instruments read to 30 but generally to 20 seconds. Occasionally 4-inch theodolites are selected for lightness at a sacrifice of capability and of distinct and exact reading. The following table gives the average weight of the transit theodolite illustrated on the last page:—
| Instrument. | Case. | Overcase. | Tripod. | ||||||
|---|---|---|---|---|---|---|---|---|---|
| 4-inch | Transit. | 11 | lbs. | 8 | lbs. | 4 | lbs. | 8 | lbs. |
| 5-inch | " | 13½ | " | 9 | " | 5 | " | 9 | " |
| 6-inch | " | 19 | " | 10 | " | 6 | " | 11 | " |
| 8-inch | " | 36 | " | 20 | " | 10 | " | 18 | " |
If with lamp extra about ¾ lb. If with striding level extra about ¾ lb.
It will be seen that the 5-inch instrument of this class with cases and tripod, say altogether 36 lbs., is really of quite as much weight as a fairly strong man can carry through a hard day's work. The 5-inch instrument is therefore becoming more and more popular with practical civil engineers, and its performance, if of good modern work, is quite equal to the 6-inch of less than half a century ago.
368.—By giving a description in detail of a transit theodolite, the general principles of a great number of other instruments, particularly those of larger dimensions, will be included, except for certain details that the specialities of the particular instruments demand. The most convenient plan to follow in this description will be to take the structure of a 6-inch transit theodolite of common construction, as it is built up from its base, piece by piece, according to the rule of ordinary structure; where more modern theodolites vary mostly from this is in having many parts shaped out of the solid, which are screwed together in the form illustrated.
369.—The Tripod Stand of a theodolite of 6 inches and under is generally made identical with that of a level, a common form being that described for a dumpy, [art. 216]. The arrangement of one turn-up leg, as shown [Fig. 63], is very advantageous for the theodolite if it is to be used on mountainous or even very hilly ground. For instruments exceeding 6 inches a framed stand, which will be described further on, is better. Some makers use a framed stand for a 6-inch instrument. The rigidity of the stand ought to be quite equal to that of the work in the theodolite, or a little in excess, and when this is attained it is sufficient. Where the stands of theodolites so often fail is from the defective construction of the tripod head, not at all from deficiency of timber in the tripod itself; and overloading this, in adding weight without attention to scientific construction, is worse than useless.
370.—In the following description of the transit theodolite the parallel plate setting-up arrangement is taken, as this is at the present time (1914) still in use in this country and in America. There is nevertheless great probability that it will not long continue to be so, as year by year the tribrach system, described [art. 233], for levels is coming more forward, both for levels and theodolites. This tribrach system the author holds to be much more scientific, and when thoroughly understood, more simple and expeditious to work with. It is also to be recommended, as there is no possible risk of strain upon the general work of the instrument, nor risk of error from distortion of the vertical axis from strain in setting it up to adjustment. A constructive drawing of a common transit theodolite with parallel plates is shown Fig. 153, of which the following is a detailed description.
