166 Mr. Barlow on the daily Variation of the [March, 



at least a minimum ; but whether this is a fixed direction during 

 the year, or whether it has any vibratory motion as the sun 

 changes its declination, or even during his daily course, is a 

 question which cannot be decided without a much longer course 

 of experiments than those I have here the honour to present. 



It is also questionable, whether the direction of this line of 

 no daily variation is the same in different parts of the world ; a 

 point on which I hope to obtain some information in the course 

 of the present year. Mr. Forster,* of H. M. S. Griper, having 

 very obligingly undertaken to repeat my experiments at Spitzber- 

 gen, during the stay of the vessel at that place for the pendulum 

 experiments ; and from which we may hope to derive some inte- 

 resting deductions, particularly in reference to the influence of 

 the direction of the solar rays ; for it is clear from the experi- 

 ments reported in the preceding table, that the amount of the 

 deviation does not entirely depend upon the moment when the 

 heat of the sun is the greatest, as has been generally imagined ; 

 for the time of the maximum deviation varies from eleven 

 o'clock in the morning to four o'clock in the afternoon, accord- 

 ing to the direction in which the needle is pointed, and to other 

 circumstances that will be mentioned in the conclusion of this 

 article. Mr. Christie's observations are also of a kind to throw 

 great light on this subject. 



Another conclusion, which I think we are justified in drawing 

 from the above experiments, is, that the daily change is not 

 produced by a general deflection of the directive power of the 

 earth, but by an increase and decrease of attraction of some 

 point situated between the north and NN vV, or between the south 

 and SSE ; for 1 cannot conceive any other hypotheses that will 

 account for two needles, situated as in these experiments, both 

 approaching and both receding at the same time to and from the 

 line of no daily variation ; nor for the total suspension or equi- 

 vocal vibratory motion of a needle when placed towards this 

 direction. 



I am sorry, that not foreseeing at the commencement of my 

 experiments, the length to which I should carry them, I did not, 

 from the first, register the temperature and state of the atmo- 

 sphere ; for from certain notes of this kind made lately, it 

 appears to me that the quantity of daily change depends in a 

 greater degree on the intensity of the solar light, than on the 

 mere temperature of the day ; although it is certain, from some 

 recent experiments by Mr. Christie, that the change of tempera- 



* I am already highly indebted to this gentleman for the accurate and satisfactory 

 observations he made during the recent voyage of H. M. S. Conway, under the com- 

 mand of Capt. Basil Hall, on the method I had the honour to propose for correcting the 

 local attraction of vessels ; and it is with great pleasure that I find he has been directed 

 by the Admiralty to continue his attention to them in the present voyage of the Griper. 

 My best thanks are also due to Capt. Hall, for the facilities he afforded in the instance 

 abovementioned, and for the judgment with which he selected the most appropriate 

 situations for submitting that method to the test of actual experiment. 



