AND PORTO SANTO. 135 



being the most expensive of any", had not M. Arago kindly 

 presented me with the simple apparatus which he used at 

 Dunkirk ; viz., a needle with an ivory scale affixed to the end 

 of it, covered by a glazed box, to prevent its being agitated by 

 any current of air, and suspended carefully by a piece of fine silk, 

 free from torsion, and descending through a hollow piece of cane, 

 with a microscope with intersecting wires for the reading of the 

 finely-graduated scale. The observations may be calculated, when 

 necessaiy to compare them with those of any other instrument, 

 by noting, that the distance from the point of suspension to the 

 scale is 12.95 centimetres, and that each line of the scale is equal 

 to 0.25 of a millimetre : for, by dividing the former quantity by 

 that of the daily movement, the quotient will be the tangent of 

 the daily variation in minutes. The results of my observations" at 

 Madeira are, that the greatest variation is at 8 a.m. and the least 

 at 2 p.m. ; and that the variation decreases with the temperature, 

 being least in the coldest season. I hope to make a more regular 

 series of observations and under more favourable circumstances at^ 



' Gambey, who furnished Mr. Ritchie's, asked me from 1500 to 2000 francs. 

 * Movement from 8 a.m., to 2 p.m., to 6 p.m., from 6 p.m., to 8 a.m. 



lines lines lines 



October 22— 2.3 1.8.3 west .9 east .least 



23 .9 „ .5 



24—25 1.08 „ .75 



25 .8 „ .5 



26—27 .4 



27 1.3 „ 1.2 



from 8 a.m., to 2 p.m. October 2Sth, 1| W. 29th, U W. 30th, li W. 31st, 

 IJW. November 1, IJ W. 4th, U W. 6th, 1 W. 13th, 1 W. 14th, i W. 

 15th, h W. 16th, ^ W. 18th, 1| W. 19th, U W. 20th. U W. 2Ist, h W. 

 22nd, 1 W. 25th, | W. 26th, i W. 27th, J W. December 5th, i W. 6th, i W. 

 7th, ^W. 14th, 1 W. I6th, 4W. 17th. | W. 20th, 1 W. 28th, | W. 



P 1701 Coetlogon, 4° W. 1720 Laval, 8° 15' W. 1758 Howe, 15° 12' W. 1761 

 Bishop, 16° W. 1769 Fleurieu, 15° W. H. M. S. Lowestoff, 16° 30' W. 1788 

 Johnston, 18 W. 



