Magnetical Dip in the United States. 19 



neous form, and has been used by Prof. Loomis to show the im- 

 perfections of my observations in a " striking light." It appears, 

 on page 89, first as -42', second corrected to -A6'.5, and again, 

 " By far the greater error here, is -46'. 5, which I obtained from 

 the first observation." Here the whole aifair is of no account, 

 because the datum on which the calculation was founded, had no 

 real existence. The same observation has been corrected by my- 

 self, and was reprinted in the January number for 1841, page 150. 

 This typographical error should have been detected, as such, from 

 its inconsistency with the other items of the same group. 



Prof. Loomis assumes in the next paragraph, that the differen- 

 ces of the readings with the face of the compass east, and with 

 it west, if they exceed a certain constant quantity by him calcu- 

 lated, and called "twice the zero error of the instrument," are 

 '* errors of observation," and upon this assumption, makes two 

 tables of errors consisting of 88 items each, at the head of which 

 stands the " typographical error.'' Here Prof. Loomis has fallen 

 into the prevailing sin of mathematicians, the hasty assumption 

 of data, which being granted, they can kill an army "by com- 

 putation." The proper zero error of the instrument arises from 

 a want of exact adjustment of the spirit level to the zero of the 

 graduated circle, and would be a constant quantity. There is 

 also another source of error, arising from want of exact adjust- 

 ment of the agate planes on which the pivots of the needle are 

 supported ; both of which errors are more or less merged by the 

 reversal of the instrument, as they would be plus with the face of 

 the instrument in one position, and minus with it in the opposite 

 position. Now this last error is a variable one, being dependent 

 upon the total magnetic force, at any place, which force is va- 

 riable. Probably Prof. Loomis intended by "zero error" the 

 conjoint effect of the relation of both the zero of graduation 

 and of the agate planes to the spirit level. A nice calculator, 

 especially when he is pointing out the errors of other people's 

 labors, should have made a distinction between them, for one is 

 a constant quantity, and the other a variable one. But Prof. 

 L. has treated the whole as a constant. He has also assumed 

 mechanical perfection in the parts of the instrument in contact, 

 the pivots and the agate planes, a condition which never exists 

 in fact. These vacillations in the various reversals which Prof. 

 Loomis has tabulated, were of course known to me, and were 

 always compared during the observations. They were apparent 



