November 30, 1888.] 



SCIENCE 



257 



Agricultural Surveys. 

 Special studies of the character and distribution of soils, and of 

 related phenomena having an immediate bearing upon the cultiva- 

 tion of useful crops, are known as agricultural surveys. The classi- 

 fication of land into groups, as desert, grazing, mining, forest, 

 swamp, etc.; the classification and properties of various soils, as 

 marl, loam, sand, clay, hammock, adobe, etc.; the study of climate 

 as related to crops ; the study of animal life, and especially the dis- 

 tribution of animal life, beneficial or injurious to agriculture, — all 

 these, with related phenomena, involve special examination and 

 study in the field, and together form the special work of the agri- 

 cultural survey. The special results of the distinctively surveying 

 part are classification and distribution, — results exhibited on maps 

 — or topographic maps — prepared for general purposes. The 

 work carried on, and the results obtained, at agricultural experi- 

 ment stations, are an important, indeed essentiaradjunct. but do 

 not of themselves constitute an agricultural survey. 



Magnetic Surveys. 

 The earth is a magnetic body. When magnetized bodies, such 

 as compass-needles (free to turn horizontally) or dipping-needles 

 (free to turn vertically), are so suspended as to yield to the influence 

 of the magnetic earth, they move in response to its magnetic force, 

 and take up certain positions or directions. These directions vary 

 with time and with place ; also the intensity of the magnetic force 

 exerted by the earth is found to be different in different places, and 

 not to be constant at the same place. 



Magnetic surveys are therefore organized to obtain observations 

 of these magnetic phenomena in various places and at divers times, 

 to study them, and to publish the results for general information, 

 the purpose being twofold : viz., first, to ascertain for the general 

 and practical use of persons using the compass, etc., magnetic dec- 

 lination or " variation of the compass " at any point of the earth's 

 surface at any time ; and, second, the observation and study of all 

 terrestrial magnetic phenomena with a view to the perfection of a 

 theory whereby all such phenomena may be predicted. As already 

 suggested, the magnetic declination varies with time and with 

 place ; the dip also varies with time and with place, and the force 

 varies with time and with place. A knowledge of the declination 

 is of immediate practical use to many people ; a knowledge of the 

 dip and intensity, however, is of less immediate practical utility. 

 But for a bettering of our knowledge of the whole subject of ter- 

 restrial magnetism, for the establishment of principles respecting 

 it, all its manifestations should be investigated by the magnetic 

 survey. 



If a magnetic survey of a State were undertaken for the purpose 

 of producing an isogonic map, or map showing by curves or shad- 

 ing the declination at all points in the State, for the practical use 

 of compass-users, different plans might be used for the purpose. 



A few stations widely separated and scattered over the State 

 might be selected, and a precise determination made at each by 

 using sufficient time, care, and delicacy of instrument. This would 

 give refined results and few stations. 



On the other hand, by using much less time at each station, and 

 less delicate instruments, more stations could be occupied, and a 

 .greater number of less precise determinations obtained in the same 

 time. This latter would have the advantage of showing distribu- 

 tion better than the former. 



For showing distribution of rainfall, it would seem that observa- 

 tions at five hundred stations, giving results accurate within one or 

 two inches, would be for many, if not for most, purposes better 

 than the results from fifty stations accurate within one or two 

 tenths of an inch, or ten times as accurate as the former. Sim- 

 ilarly, if for the purpose of constructing an isogonic map we have 

 our choice between determinations of declination at one hundred 

 stations accurate within one or two minutes, and determinations at 

 one thousand stations accurate to within ten to twenty minutes, it 

 may not be easy to decide which to choose. Surveys have been 

 tried in the United States by both methods, neither of which com- 

 pletely satisfied the parties conducting them. The latter method 

 (has not been sufficiently tested experimentally to prove its quality. 

 But, for the purpose of producing an isogonic map for a given 

 €poch, the writer considers it better to go rapidly over the area to 



be mapped, securing a very large number of observations at many 

 stations and of only a moderate degree of accuracy, than to have 

 highly refined and precise measures made at only a few stations. 



Nautical Surveys. 



As the object and purpose of a topographical survey is the produc- 

 tion of a map, so the object and purpose of a nautical survey is the 

 production of a chart. Such has been the only purpose until recent 

 years, when the ocean, its movements, its inhabitants, its depths, 

 have become subjects of special study. This special field, under 

 whatsoever name included, — whether ' ocean physics,' ' thalassog- 

 raphy,' or 'physical hydrography,' — is only indirectly and remotely 

 connected with nautical surveying as usually understood : hence we 

 may regard the term ' nautical surveying ' as embracing ordinary 

 hydrographic and physical hydrographic surveys ; the object of the 

 first being chart-making, and of the second an investigation of the 

 oceans and great water bodies for purposes connected more or less 

 indirectly with navigation. The chart produced by the nautical 

 surveyor is usually supplemented by some form of directory, or 

 hand-book, or coast-pilot, giving certain data useful to the mariner 

 in addition to that afforded by the chart. 



Similarly, the map produced by the topographic surveyor maybe 

 usefully supplemented by some form of guide-book, gazetteer, or 

 geographical dictionary, affording certain useful data supplemental 

 to that contained on the maps. 



Tidal observation and current observation form proper parts of 

 ordinary nautical surveys. The purpose of such observations is the 

 immediate and direct one of aiding navigation : hence the selec- 

 tion of stations, and the character and extent of the observations, 

 will be made to accomplish this purpose. If, however, the tidal 

 observations are made for obtaining data whereby the theory of the 

 tides may be perfected, if the current observations are made to dis- 

 cover the general laws of oceanic circulation and their results, then 

 these considerations will lead to a choice of stations and methods, 

 and amount of observation, which gives promise of best serving 

 that purpose. 



We have now briefly reviewed and commented on six species of 

 information surveys, — geodetic, geologic, topographic, agricultural, 

 magnetic, and nautical. These surveys are all national works, cov- 

 ering wide areas and long periods for their execution. Moreover, 

 most of them cannot be done once for all, but must be repeated 

 from time to time. The best and completest and most perfect 

 work of the eighteenth century does not satisfy the demands of the 

 nineteenth ; and the surveys of the nineteenth will serve their pur- 

 pose, even if the twentieth century finds it necessary for its pur- 

 poses to make new and better surveys. The object of a survey is 

 not the attainment of the highest possible precision. Great accu- 

 racy is needful for the accomplishment of certain purposes. Such 

 accuracy, however, is not itself the purpose : it is only the means to 

 the end. What the purpose of information surveys are, we have 

 tried to set forth. If correctly set forth, these purposes will furnish 

 the criteria for judging of the precision which should be striven for 

 in any work ; and it will thus appear, that, if an accuracy less than 

 the greatest will serve a specified purpose, the greatest accuracy 

 and the cost of securing it are unnecessarj'. The work done should 

 be sufficiently good for its purpose. 



Boundary Surveys. 



The second great division m ourenumeration consists of boundary 

 surveys, and these may be conveniently grouped under two heads : 

 first, those lines which separate communities or jurisdictions, such 

 as towns, counties, states, and nations ; and, second, property 

 boundaries, or boundaries of private ownership. Perhaps the terms 

 ' public boundaries ' and ' private boundaries ' might be used to in- 

 dicate these two groups. 



Boundar)' surveys differ from information surveys in this : they 

 deal with lines, information surveys deal with areas. The problems 

 presented by the boundary survey are generally more definite and 

 explicit than those of surface sur\'eys, and there is correspondingly 

 less opportunity for display of judgment and skill in their conduct. 



The purpose of a boundary- survey is to mark out a line on the 

 earth's surface. As the marks placed by such survey are subject 

 to loss, removal, or obliteration through neglect or malice, it is es- 



