104 Dip and Variation of the Magnetic Needle. 



potheses on which they are computed, or errors of a different 

 kind? They cannot be principally errors of observation. At 

 several of the stations where the differences are greatest, such as 

 Philadelphia, Columbus, Cincinnati, and Madison, the dip has 

 been repeatedly observed by different individuals, and nearly uni- 

 form results obtained. Do they indicate errors of the hypotheses 

 on which they are computed ? These hypotheses are that the 

 isoclinal lines over a limited territory are straight, parallel, and 

 equidistant. Doubtless each of these hypotheses may be erro- 

 neous, and if the lines are extended over any very considerable 

 portion of the globe, will be palpably so. But how great are 

 these errors within the limits of my several tables ? The obser- 

 vations themselves afford data for estimating the errors. The 

 error arising from curvature and want of parallelism of the isocli- 

 nal lines, will appear in the unequal mean differences under the 

 several meridians ; and that arising from the lines not being equi- 

 distant, will appear in the unequal mean differences under the 

 several parallels. We have already seen that for each of the 

 tables, these errors are much less than the mean of the differences. 



plored. But Prof. Locke claimed for his observations an extraordinary degree of 

 accuracy; an accuracy much greater than I could claim for my own, and greater 

 than I had seen claimed by any former observer. In his own language he expected 

 his results would be received with ^surprise;' and I thought it not amiss to en- 

 quire whether the individual observations gave indications of this extraordinary 

 precision. The result was the development of anomalies of precisely the same 

 kind as had perplexed me in my own observations. These anomalies doubtless 

 in a measure neutralize each other by being alternately positive and negative, and 

 are still further reduced by division, so that the mean results may be valuable in 

 spite of them. I have lately had an opportunity of comparing my own observa- 

 tions with Prof. Locke's, by going upon several of his localities. The following 

 is the result of the observations. 



At none of the above places were the stations employed identically the same 

 except at Cincinnati, and here I have taken for Prof Locke's result the mean of a 

 year's monthly ob.servations. My own results are upon an average not quite one 

 minute less than Prof. Locke's. I am satisfied then that the two series of obser- 

 vations may be compared, as if they were all made with one instrument. 



