20 HYDROGRAPHICAL SURVEYING [chap. i. 



Lifting it with the finger will always assist it to run round 

 easily. 



5. The fact of the tangent screw being run out nearly to the 

 end of the thread, will frequently account for a theodolite not 

 coming back to zero if the thread is worn. This may occur in 

 the case of the tangent screw of either the upper or the lower 

 plate ; in such a case the tangent screw should be turned 

 until it nearly butts, the last few threads being probably 

 less worn. 



Putting in An operation the nautical surveyor has frequently to per- 



w^e^s. form is replacing the wires, or rather cobwebs, of his theodohte 

 telescope. 



For this purpose catch a garden spider, as a house spider 

 does not spin his rope taut enough. Having cut some holes, 

 say 2 inches square, in a strip of cardboard about 3 inches 

 wide, place the spider on it, and shake him off. As he throws 

 out liis web in falling, twist it up on the cardboard so as to 

 cross the holes, and lay it on one side. 



Having taken the diaphragm from the telescope, and scraped 

 off the old balsam, lay it on the table and place the smallest 

 drop of Canada balsam on its edges. With the aid of a mag- 

 nifying glass, place the cardboard across it, in such a manner 

 that the web will lie in the notches cut in the diaphragm, when 

 it ^vill adhere to the balsam. 



A silk thread, exceedingly fine, is an excellent substitute for 

 cobwebs, and is indeed superior in some respects, being more 

 easily inserted and stronger. 



Measuring Heather gives a good description of measuring angles with 



wiUi^the ^^® theodolite, to which we will add, that, 



Theodo- Regard must be had to the purpose for which the angles are 

 to be taken, in setthng how many times, and in what manner, 

 the angles shall be repeated. An error of one minute will 

 make no perceptible difference when plotted, unless the line be 

 very long, say 5 feet. All objects, therefore, that are simply 

 to be plotted, and do not come into the triangulation, can be 

 taken round once with zero at 360°, and a second round taken 

 after with another zero, say 100° for convenience' sake, simply 

 for the purpose of making sure that there are no gross errors, 

 as no theodolite in adjustment should give an angle in error 

 exceedincj one minute. 



