198 



differed from that of Mr. Barlow, for instead of applying a magnet 

 in the horizontal plane of the needle he placed two in the line of the 

 dip, having their poles in the reverse position to those of the needle. 

 In describing the results of his experiment, Mr. Christie calls the 

 south pole of a magnet that which points to the north pole of the 

 earth, so that the north end is the south pole, and the south end the 

 north pole of his magnetic needles. In stating the deviations from 

 any point considered as zero, those which he places in the direction 

 of the sun's apparent daily motion are considered minus, and those 

 in a contrary direction plus, whatever may be the position of the 

 needle. The results of Mr. Christie's observations are given in the 

 form of tables, the first of which extend from March 26 to March 31 , 

 and exhibit manifest deviation towards the east before eight in the 

 morning, and the greatest westerly deviation about one P.M. In a 

 second series of observations, with another needle, the directive 

 force of which was to the power as T63 to 1, the times of the 

 greatest easterly and westerly deviation agreed with the former, but 

 the easterly was greater compared with the westerly. In a third 

 series the suspension of the needle was improved, and the magnets 

 were so adjusted as to render its directive force as 0' 68 to 1. With 

 this needle the observations were made nearly every hour, from the 

 5th to the 12th of April, and the mean results give the greatest 

 easterly variation at 7 h 45 m , the time of no deviation at 9" 26 m ; the 

 greatest westerly deviation took place at l h 24 m , the time of zero 

 being 6 h 40 m , and the total daily variation amounting to 3 41'. 

 After adverting to the influence of the weather, to irregularities re- 

 ferrible to the electric state of clouds, and to other causes of ano- 

 malies in prosecuting his inquiries, the author proceeds to determine 

 the results of a numerous series of observations made as the former, 

 but in which the position of the needle was somewhat altered, as 

 well as that of the bar magnets ; and to examine into the daily 

 changes which take place at the points at which a needle is retained 

 in equilibrio by two bar magnets, which he is led to believe arise 

 from an actual change of intensity in the terrestrial forces, as well as 

 from a change in their directions. 



The author concludes his paper with remarks upon the influence 

 of temperature in diminishing the power of the magnets, and infers 

 that solar heat exceeds any other in producing such an effect ; but 

 that changes of temperature are not the only cause of the variations 

 which take place in the points of equilibrium, is shown by their oc- 

 curring independent of the temperature of the magnets. 



On Fossil Shells. By Lewis Weston Dillwyn, Esq. F.R.S. In a 

 Letter addressed to Sir Humphry Davy, Bart. P.R.S. Read June 

 5,1823. [Phil. Trans. 1823, p. 393.] 



In describing the shell fish supposed to yield the Tynan dye, Pliny 

 has adverted to its power of boringthe shells of other fish ; and Lamarck 

 says that all mollusca, whose shells have a notch at the base of their 



