Proceeding's of'ttte Royal Society of Edinburgh. 379 



Of these results, those obtained in the Alps, and by needle No. 1, 

 are the most trust-worthy. From the variation due to latitude and 

 longitude, the direction of the isodynamic lines is easily found. In 

 the Alps, they form an angle of about 78 with the meridian, to the 

 E. of north, nearly coinciding with the geological axis of that part 

 of the chain. The results in the Pyrenees are of a more doubtful 

 character, the observations having been made over a very small 

 area of country, and exhibiting some anomalies. They would ap- 

 pear to indicate a direction from N. of W. to S. of E., which coin- 

 cides with the mineralogical axis, and does not agree with Hans- 

 teen's map. 



With regard to the effect of height, the author first consider* 

 the evidence already brought to bear on the subject, which he con- 

 siders as almost quite inconclusive, from the imperfect nature of 

 the data, and the limited extent of the induction. The result at 

 which he has arrived as the most probable, from the combination of 

 all his experiments, is a diminution of T (j^th part of the intensity 

 for 3000 feet of ascent ; a quantity so small, that it can only be ex- 

 pected to be discovered by combining a great many observations. 

 The sum of the heights of the stations to which he has carried the 

 Hansteen apparatus, amounts to above 160,000 feet, or 30 vertical 

 miles. 



In l\\cjifth and last section of his paper, the author quotes his 

 observations on magnetic dip, which he has determined (with a 

 small instrument) at a considerable number of points ; and by com- 

 bining the results in the same manner as before, be has endeavoured 

 to approximate to the position of the lines of equal dip in the 

 Alps. 



2. Notice respecting a New Reflecting Microscope. By Mr 

 Guthrie. 



Mr Guthrie modifies Amici's microscope, by removing altogether 

 the plane speculum, and placing the object to be viewed in the axis 

 of the tube. This arrangement is to the microscope what Sir W. 

 Herschel's is to the reflecting telescope. In order that the object 

 may be properly illuminated, the part of the tube next the mirror 

 is wholly removed, and three pillars substituted for it, to one of 

 which the stage for the object is attached, and regulated by an ad- 

 justing screw. 



Some observations were made by I)r Martin Barry on Uni- 

 ty of Structure in the Animal Kingdom. 



