OUR GUARDIANS ON THE DEEP 



G65 



showing the time when they cross the 

 meridian of a place of known longitude. 

 A simple calculation, then, shows the dif - 

 ference of longitude of known and the 

 unknown positions. 



The latitude of a place is fixed also by 

 observing the stars, when the Survey 

 does its finest work. With a zenith tele- 

 scope it notes the exact time when a pair 

 of stars culminate, and after making al- 

 lowances for refraction, temperature, 

 and the like the determination becomes 

 merely a mathematical one. 



In making a survey, after the longitude 

 and latitude of the starting point has been 

 ascertained, a base-line is laid out. Its 

 direction, with reference to the meridian 

 of its starting point, is fixed with great 

 accuracy ; the line must not turn to the 

 one side or to the other more than 1/33 

 of an inch to the mile. The instrument 

 with which this determination is made is 

 known as the theodolite, and consists of a 

 small telescope mounted on a horizontal 

 graduated circle. The lines on this circle 

 are so finely engraved as to be invisible 

 to the naked eye, and the operator reads 

 each measurement by three different mi- 

 croscopes of high power, each of which 

 must tell him identically the same story. 

 Furthermore, he repeats this process of 

 measuring and reading many times, so as 

 to make sure he has avoided mistakes. 



The length of the base-line is fixed as 

 accurately as its direction, else all the 

 work that follows would have no value 

 of exactness. In the past these lines were 

 measured with various kinds of appa- 

 ratus, their accuracy being checked by 

 comparison with a five-meter bar. This 

 bar was a little more than five meters 

 long, but the measuring part of it was 

 the distance between two lines, invisible 

 to the unaided eye, engraved upon irid- 

 ium plugs inset in its surface. While in 

 use, the bar was kept at a constant tem- 

 perature by melting ice packed around 

 it. The trough supporting the bar was 

 mounted on a carriage moving upon a 

 track built for the purpose. The succes- 

 sive positions of the ends of the bar were 

 determined by high-powered microscopes 

 firmly attached to heavy posts solidly 

 planted in the ground. 



This method, slow and expensive, even 



when used to check the work of sim- 

 pler apparatus, has in late years given 

 place to the invar-tape method. Invar 

 tapes are made of an alloy of nickel and 

 steel, and their lengths are practically ex- 

 empt from the influences of heat and 

 cold. They are used in connection with 

 spring balances, which determine the 

 amount of tension on the tape at the mo- 

 ment of measurement. This method is as 

 cheap as it is accurate and permits much 

 more frequent base-lines, and therefore 

 there is less liability to error (see page 

 662). 



DIFFICULTIES OF THE) WORK 



The position of the starting point de- 

 termined, the length and direction of the 

 base-line measured, the distances be- 

 tween hundreds of other points are fixed 

 by triangulation. The surveyors take 

 one line of known length, and from its 

 two ends observe two other directions to 

 a common meeting point, thus making a 

 triangle. By measuring the angles of the 

 triangle they can determine the distance 

 between the two ends of the base line 

 and the third point more rapidly than 

 if they had taken a standard meter-bar 

 packed in melting ice and laboriously 

 measured the entire distance up hill and 

 down dale, across river and over moun- 

 tain peak. 



In some of the triangulation in moun- 

 tainous country the lines joining adja- 

 cent stations are frequently as much as a 

 hundred miles long, and for such dis- 

 tances special signals have been devised. 

 In daylight observations are made on the 

 heliotrope, a reflecting mirror which re- 

 flects the rays of the sun to great dis- 

 tances. The most satisfactory signal, 

 however, is a high-powered automobile 

 headlight used at night. Some of these 

 lamps in the atmosphere of Arizona have 

 shown distinctly through the telescope of 

 the theodolite over a distance of 120 

 miles (see page 669). 



In triangulation work high mountains 

 are desirable, as they more than atone 

 for the difficulty involved in scaling them 

 by the great area of country they spread 

 before the observer. When work is done 

 on level or nearly level ground or in 

 heavily wooded country, it is necessary 



