460 



SCIENCE. 



[Vol. VII., No. 172 



to pricks and other punctiforin pain-excitants. 

 Goldscheider admits, therefore, not only the ex- 

 istence of nerves exclusively devoted to percep- 

 tions of temperature, but specific nerves for heat 

 and cold. The sensibility of the surface of the 

 body to temperature presents great topical varia- 

 tions, and is directly dependent in any region 

 upon the number and intensity of the tempera- 

 ture-points, — that is to say, upon the local wealth 

 of temperature-nerves, — and go hand in hand with 

 the distribution of the great nerve-trunks. Gold- 

 scheider also differentiates in the skin nerves of 

 general sensation and specific pressure-nerves. 

 The latter terminate in certain points of the skin 

 which are not only especially sensitive to very 

 delicate contact, but contain also peculiar organs 

 which excite a granular sensation on pressure. 

 The pressure-points are arranged after the same 

 fashion as the temperature-points, but are in gen- 

 eral much more closely aggregated. Both they 

 and the temperature-points supply us with infor- 

 mation in regard to locality. 



— Any one may become a member of the 

 Roman alphabet association, to which reference 

 is made in the article in this number on ' The in- 

 tellectual movement in Japan,' by the payment of 

 an annual fee of one dollar. All donations should 

 be addressed to Roma-ji-kai, Tokio, Japan. 



— The dredging-machinery for the excavation 

 of the Panama canal is exceedingly powerful. 

 One of the dredges excavates 3,300 cubic metres 

 per day, and there are two others which excavate 

 800 and 1,000 cubic metres. Besides these, there 

 are a number of smaller ones in operation, in all, 

 capable of excavating 37,000 cubic metres per 

 day. It is reported that during the month of 

 February, upwards of 1,100,000 cubic metres were 

 excavated. 



LETTERS TO THE EDITOR. 



*** Correspondents are requested to be as brief as possible. The 

 writer's name is in all cases required as proof of good faith. 



On a geodetic survey of the United States. 



I have been often asked why a geodetic survey 

 and triangulation is the only mode of surveying 

 a large area with precision, and why such slow and 

 tedious methods are requisite for needful accuracy. 

 This paper is an attempt to show, in popular lan- 

 guage, both the processes themselves and their neces- 

 sity : as also why congress should act upon the 

 repeated recommendations of the national academy, 

 and carry out its views. 



To many of the habitual readers of Science, this 

 letter will appear to deal with elementary matters 

 which they may be assumed to know. To another 

 large and equally earnest class of readers, it may 

 convey useful information. Possibly it may help 

 forward the end sought for ; and to this every time 

 lover of science will cry 1 God speed.' 



Any survey of a small area, as a farm, plantation, 



or township, may be made by any of the usual 

 methods adopted in ordinary land-surveying, where 

 the area covered by the survey is treated as a plane 

 surface. 



The compass and Gunter's chain of sixty -six feet 

 are the usual surveying-instruments in this country. 

 They are liable to serious error. Lack of knowledge 

 of the true local magnetic variation of its secular 

 change from year to year, and of its diurnal change 

 between morning and afternoon, with the always 

 impending possibilities of special local attraction at 

 or near the place surveyed, are among the difficulties 

 attending the use of the compass. The chain 

 stretches with use, and changes its length with the 

 seasons and their varying temperatures, and is often 

 carelessly carried by men little accustomed to pre- 

 cise methods. It is not too much to say that any 

 land worth fifty dollars an acre is too valuable to be 

 surveyed with a compass, and any record of such a 

 survey is likely to become a fruitful source of future 

 litigation. The best of such surveys are but approx- 

 imations to the truth. 



Errors from these approximate measurements are 

 cumulative. When such surveys are extended over 

 large areas, as upon our public lands, serious conse- 

 quences follow, involving present and future doubt 

 and litigation as to boundaries. This is already ap- 

 parent in the west. It will become more so in the 

 future as land increases in value. 



The necessity for greater precision in original pub- 

 lic-land surveys, and for means of ascertaining and 

 checking errors already existing, has been forcibly 

 stated in a report to congress on the survey of 

 the territories, by the National academy of sciences 

 in November, 1878, printed in 'Misc. doc. No. 5, 

 house of representatives, 45th congress, 3d session.' 

 The report of the academy, and the very strong let- 

 ter of Major J. W. Powell, which forms a part of it, 

 fuily describe the character and consequences of the 

 errors alluded to. It also sets forth the true remedy 

 as only to be found in a method of survey which 

 should be as nearly infallible as scientific skill and a 

 laborious and careful application of well-known 

 principles could make it. 



This method, as practised for two centuries by 

 civilized nations, consists of a system of triangles, 

 starting from and proceeding toward certain base- 

 lines, measured with every possible care with ap- 

 paratus specially devised to either entirely elimi- 

 nate, or to reduce to a minimum, every source of 

 error, whether physical or mechanical, which might 

 vitiate the resulting length of the measured line, or 

 cast a doubt upon its precision. 



Apparatus of this nature is now constructed and 

 used, in the U. S. coast and geodetic survey, of such 

 precision that the average probable error of the two 

 primary bases last measured with different apparatus, 

 constructed on different principles, is, roughly, about 

 one twelve-hundred-thousandth part of the lengths 

 of the measured lines. 



The exact length of the base being ascertained, 

 and a system of triangles built upon it adapted to 

 and covering the country to be surveyed, the lengths 

 of all the other sides of the triangles in the system 

 are inferred from the familiar theorem that " every 

 triangle has six elements or functions, — viz., three 

 sides and three angles, — any three of which being 

 known (one being a side), the other unknown ele- 

 ments may be computed " with a degree of precision 

 of the same order as that of the known elements. 



