.GEOGRAPHICAL LATITUDE. 779 



of whose circumference is accurately kuown, is rolled over the way, 

 thus producing the effect of a continuous rule. This has been applied 

 with good result, though it has not been used frequently. As the work 

 of measurement can not be complet»'d in one day, it is necessary to 

 mark with care the place of quitting at evening, for commencing in the 

 morning. This is done, when measuring with rules, by dropping a 

 plumb line from the end of the last rule placed at evening and mark- 

 ing the spot where it falls on a plate sunk into the ground, which is 

 then carefully covered, so that neither storm nor wandering beast can 

 change its position during the night. With all these precautions and 

 the most careful noting of every circumstance that can in the least in- 

 fluence the result, the line is generally measured twice, and as a con- 

 trol of the accuracy of the measurement, there is often measured a sec- 

 ond line distant from the first. 



Added to this is the labor of running the net of triangles between 

 the ends of the line to be surveyed and connecting it, as accurately as 

 possible, with the base line. The points for the angles of the triangles 

 having been selected, there are the necessary preparations for observa- 

 tions 10 be made, including, where necessary, the building of wooden 

 towers of greater or less height. As the summits of mountains are 

 frequently selected therefor the difficulties of procuring all necessaries 

 of life and for the work are thereby materially increased, while the 

 violent action of the elements often disturbs and sometimes destroys tlie 

 work of a long period. The stations established, it remains to measure 

 the angles, not only once, but a number of times; and not only two 

 angles of each triangle (which theoretically would be sufficient), but in 

 order to insure the greatest possible accuracy, all three arc measured 

 with extreme care. For this work, the improvement in signals and 

 instruments has been immense. 



These surveys have led to some valuable observations on the varyin.'i; 

 conditions of the atmosphere. Perhaps the most interesting of these 

 after those relating to refraction are those which prove a periodic dis- 

 turbance or movement in the atmosphere, even on so called still days. 

 There are four phases daily, two of agitation and two of quiet. The 

 period of greatest quiet or of least motion is in the night, for which 

 reason it has often been found advantageous to make the observations 

 at that time, using artificial lights as signals.' 



After the angles have been measured it is necessary to calculate the 

 difference in altitude of the various observing points, in order to reduce 

 all to the same plane, when these, together with the base line itself, are 

 reduced by elaborate mathematical calculations to the level of the sea. 



' An excellent illustration of this was given in carrying the French-Spanish merid- 

 ian to the Balearic Islauds. "Die Dreyecks-Seite von Sierra Mordla bis Silla de 

 Torellas auf Mayorca betriigt 93080 Toiseu. Ein einziges Vi'erer/^crf auf Mayorca hat 

 man auf der spanischen Kiiste niit Fernrohrcn drey Stuiideu lang gesehen." Zach, 

 Mon. Cor. Junius, 1803, p. 569. As early as tlie time of Picard it was found advan- 

 tageous to use fire signals at night. See Picard, Mesure de la terre, p. 9. 



