Dr. J. Reade on a permanent Soap-bubble. SI 5 



51. The lines of equal dip and equal horizontal intensity 

 being known, the direction of the lines of equal total intensity 

 may be deduced geometrically. I am, however, too well aware 

 of the great uncertainty which a small error in the elements 

 produces, to attempt to assign a result which might prove very 

 erroneous indeed. 



Postscript. — Since this paper was written, and the results 

 made public, a suggestion has been made in a quarter entitled 

 to attention, as to now far the apparent diminution with height 

 may be due to the hour of the day at which observations at 

 great heights have usually been made. I have already stated, 

 that I have attempted no correction for the hour of the day, 

 owing to the want of accurate data, but I thought it worth 

 while to inquire how far there was any general ground for 

 such an explanation of the observed difference. I accordingly 

 divided my observations into 18 series above 4-000 feet, and 

 22 below that height. I found that the mean hour at which 

 the former were made was ll h 12 m , the latter at 12 h 42 m . 

 According to the best observations, the intensity would be 

 somewhat less at the former period than the latter, and would 

 so far give a false indication of diminished intensity with height. 

 But the variation for l h 40 m would undoubtedly be trifling, 

 compared even with the small variation which the preceding 

 paper assigns for 5110 feet, which corresponds to the mean 

 difference of height for the two series, the mean height for the 

 first being 7160 feet, and for the second 2050. 



[Prof. Forbes's paper, as printed for the Transactions of 

 the Royal Society of Edinburgh, is accompanied by a " Map 

 of the Central Alps," shewing the lines of equal horizontal 

 magnetic intensity, and equal dip, in 1832. As the magnetic 

 lines in this map have been projected entirely from the obser- 

 vations recorded and discussed in the paper, we have deemed 

 it unnecessary to copy it. — Edit.] 



XL VI. On a permanent Soap-bubble, illustrating the Colours 

 of thin Plates. By Joseph Reade, M.D.* 



IVTO subject in natural philosophy has more engaged the 

 ^■/j attention of the learned than the discovery of a perma- 

 nent soap-bubble. Mr. Boyle, Dr. Hooke, and Sir Isaac 

 Newton were among the first, Dr. Herschel and Sir David 

 Brewster among the last experimenters. After such characters 

 it may appear presumptuous to enter the lists unassisted by 



• Communicated by the Author. 



