55f? Mr Barlow on the Diurnal Variation 



" These comparative observations on the intensity of the dipping 

 and horizontal needles, were made with a particular object in view, 

 which it will be proper to explain before giving the details. 



" It was found by observation, that the intensity of the horizon- 

 tal needle was hourly varying. This appeared by the results al- 

 ready given ; but it was doubtful whether this variation of horizon- 

 tal intensity of a needle, proceeded from an actual variation in the 

 intensity of the terrestrial magnetism, or from a variation in the 

 amount of its direction, as indicated by the dip itself. 



'^ The power of the horizontal needle varying as the cosine of the 

 dip, a change to the amount of a few minutes in the dip, at places 

 where it is very great, would be sufficient to explain all tiie varia- 

 tions of intensity observed in the horizontal needle, without suppo- 

 sing any change to have taken place in the intensity of the terres- 

 trial magnetic force. 



** The variation in dip, however, \i! it did occur, was too small to 

 be detected by direct observation ; and I failed also to render it sen- 

 sible by the application of magnets, as stated in a former communi- 

 cation. 



" My object, therefore, in making the experiments contained in 

 the following table, was to ascertain, by several series of vibrations 

 made with the same needle, mounted alternately as a dipping nee- 

 dle, and as a horizontal one, whether or not a corresponding varia- 

 tion of intensity would manifest itself in these two positions respec- 

 tively, as ought to be the case, if the diurnal changes of intensity in 

 the horizontal needle proceeded from a general change of intensity 

 in the terrestrial magnetic power. But, on the other hand, if the 

 force indicated by the dipping needle should be found to remain 

 constant, then it would be equally clear, that the variations of inten- 

 sity in the horizonta-l needle proceeded from an actual change of 

 dip only. 



"As this question is of considerable importance in the theory of 

 terrestrial magnetism, I regret that I had not an opportunity of 

 making a more extended series of experiments of this kind ; but, as 

 far as they go, they certainly appea;- to indicate, that the alterations 

 of intensity in the horizontal needle, are due rather to a daily change 

 in the amount of the dip, than to any variation in the general inten- 

 sity of the earth's magnetic force, although some change in this also 

 is observable by vibrations of the dipping needle. This explanation 

 of the cause of the change of horizontal intensity, it may be remark- 

 ed, is consistent with the observations made in Em*ope, which like- 

 wise shew an alteration of intensity in the horizontal needle during 

 the day, but in a much less degree than at Port Bowen. Now, if 

 the variation in question really proceed from a change of dip, to the 

 amount of three, four or five minutes of a degree, the change of in- 

 tensity in the horizontal needle will be less and less obvious, as the 

 dip decreases j but if it proceed from a change in the actual inten- 

 sity of the earth's magnetism, it ought to be constant in all parts of 

 the world, which is contrary to observation. 



" The following are the results of these experiments. The table is 

 divided into two parts , the first contains the observations on the 

 times of vibration of the needle in its horizontal position j and the 

 second, those on it when used as a dipping needle. In the first co- 

 lumn of each part, is inserted the day of the month ; in the second. 



