792 SEC. 15. GEOGRAPHY. 



(10.) Topographical Map of Spain. 



The map of Spain on a scale of g * 00 is composed of about 1,078 sheets, 

 engraved in five colours, with numerous details. The sheets comprising 

 Madrid, Colmenar, Viejo, and Yetafe are already engraved. 



The general plan of the levellings projected for Spain (plate 20) compre- 

 hend, besides the line of Alicante to Sautander, passing by Madrid, other lines 

 which, starting from Madrid run one to the N.E. which is to connect itself in 

 Perpignan with the French lines, another towards the south, which will serve 

 to determine the difference of levels between the observatories of Madrid and 

 St. Ferdinand, and two others which starting from Madrid will go to Lisbon 

 and Oporto. The remaining lines will complete this plan, and bring out 

 the levels of the capitals of provinces and towns of importance. 



The sj stern of lines is so disposed that it offers numerous polygons in order 

 to establish conditions which may give greater precision to the results. 



In conformity with the instructions of the Commission appointed to carry 

 out this map, the levelling of precision will follow in general the lines of 

 communication, such as railroads and bye-roads of every description. The 

 levelling will be duplicate, and earned out from the centre by different 

 observers, and with different instruments. The calculations will be taken 

 from Madrid. The greatest error admissible, deduced from a double levelling, 

 is not to exceed 4 millimeters per kilometer, and in general will be less than 

 5 mm VH, E being the number of kilometers. 



The general lines will be divided into independent sections, and these into 

 portions of a kilometer, with benchmarks at the extremities of the section, 

 important towns, &c. 



In August 1871, the levelling of the line between Alicante and Madrid was 

 begun; touching Madridejos at its base, 540 kilometers were levelled in 916 

 days, of which only 451 were useful. The number of stations verified was 

 11,782. 



The line from Santander to Madrid was levelled in 1873 in the same 

 manner, and this will determine the difference of level between the ocean 

 and Mediterranean. 



Without taking into account the datum points of the seaport towns, a 

 standard datum has been established irrespective of mean sea level. This 

 point is situated at the observatory of Madrid, although, considering its 

 geological conditions, the hill upon which it is placed does not offer the 

 securities which might be desired. Four others have been placed in different 

 parts of Madrid. 



(11.) Experiments made with the Base-measuring Apparatus 

 belonging to the Commission of the Map of Spain. Madrid, 1859. 



The authors, Don Carlos Ibaner and Don Frutos Saavedra Meneses, were 

 appointed in 1853 to design the system of microscopes and bars with which 

 the base of the geodetic network of the Peninsula was to be measured. 

 They went to Paris to superintend the construction of an apparatus of their 

 invention, intrusted to M. Brunner. 



This apparatus principally consists of a platinum bar, which forms a 

 metallic thermometer with another of latteeu ; they both rest on an iron bench 

 placed upon movable supports resting upon wooden tripods. Various micro- 

 meter microscopes are placed in the centres of graduated circles fixed in 

 other tripods, and these divide the base which is to be measured into small 

 intervals, the lengths of which are determined by placing the bar between 

 every two microscopes, and observing the divisions on the platinum and 

 latteen. An eyepiece and two lenses are used to range the tripods of the 



