MEASURING THE EARTH'S SURFACE. 249 



The same is done for noting the variation in temperature by measur- 

 ing the space between the ends of the iron and zinc rods in each case. 

 In order to leave no cause for error, the two ends of each rod are cut 

 to sharp edges and made one vertical and the other horizontal, and the 

 horizontal end of one rod faces the vertical end of the other. The 

 same is done for the zinc and iron. When one of the cases is moved 

 forward and placed in position ahead of the others, all the measure- 

 ments between any two rods are repeated in order to detect whether 

 any of the other rods has moved while the other was being placed in 

 position. Spirit-levels are on each case, and their variation is carefully 

 measured with microscopes. All the data are registered, and have 

 afterward to be carefully gone over, and all compensations for error, 

 temperature, etc., duly allowed. This apparatus is known as Bessel's 

 base-measuring apparatus, the chief feature of which is the metallic 

 thermometer with iron and zinc. 



The length of a geodetical base varies according to the area to be 

 covered by the net and to the possibility of finding a good ground for 

 laying such a base. From five to ten miles would be considered a 

 good geodetical base, one longer than the higher figure not being ad- 

 visable on account of the possible increase in the inevitable error of 

 such measurement. Even bases of about two miles have been meas- 

 ured, but these could not be used for very large triangulations, and are 

 more intended as a check on other measurement and as a means of 

 compensating errors. The ground has to be carefully leveled before 

 the base can be measured. The two end-points of the base are marked 

 by stone monuments which can be seen at a distance, and the whole 

 length is divided into so many sections, each of which constitutes a 

 day's work, stone and metallic tablets being laid and the line marked 

 in advance before the actual measurement can be undertaken. 



It is not necessary that the base should be actually measured be- 

 fore the real triangulation work — that is, the measuring of the angles 

 at each of the stations of the net — is begun. The work of measuring 

 a base is necessarily very slow. All apparatuses have to be carefully 

 tested before being actually used in the field. Each of the rods has 

 to be subjected to a series of experiments at different temperatures, in 

 order to determine the actual expansion of the iron by comparison 

 with that of the zinc. The actual measuring in the field occupies one 

 or two months' time, and may be longer if the weather is not favora- 

 ble, the whole distance, divided into sections, each of which constitutes 

 a day's work, being measured at least twice, once in each sense. Five 

 or six experienced operators are required, besides a number of assistants 

 to do the menial work — such as carrying the rods forward, etc. The 

 calculations are, later on, done in the office, and are in themselves a 

 very slow and exhaustive process. Taking all together, the time re- 

 quired is little short of one year, including the preparatory experi- 

 ments and the calculations. 



