498 PHILOSOPHICAL TRANSACTIONS. [aNNO 1720. 



measure by it the different levels of places, too remote to be come at by the 

 ordinary instruments for levelling, with the certainty that may be desired. For 

 this purpose, let there be provided two small glass canes, as near as can be 

 similar, being very little taper or smaller at the closed end, so that being in- 

 verted, the Mercury may be suspended in them at the height it ought to have 

 at the time of the experiment. Let that height be duly noted, and then 

 ascending the monument, or some such edifice, where the ascent may be 

 exactly measured, let the scales annexed be divided into parts by the descent 

 of the Mercury, at every 10 feet, in both the pendent barometers, which I 

 conceive may be so chosen as to make the divisions very distinct and sensible. 

 These being thus prepared, when it is desired to take the level of two distant 

 places, let one instrument be fixed in the lower place at a time when the 

 Mercury has the same height as when they were first inverted and graduated; 

 and let the other be carried to the higher place, where it will be found to stand 

 at that division which answers to the elevation of that place above the other, 

 which had before been found by measure in ascending the monument. Thus 

 may QO feet ascent, which makes only one 10th of an inch of Mercury, be re- 

 presented by 2 or 3 inches, or a space capable of being divided into QO parts : 

 whereas, if the distance of the two places be 20 miles,, a minute of a degree is 

 equal to above 30 feet ; and by the usual sights, whether telescope or other- 

 wise, of the water levels, I fear it will be very hard to convey a true level 

 without a greater error than one minute in the whole. 



Observations on the Variation of the Needle made in the Baltic, Anno 1720. 

 By Mr. fVilliam Sanderson. N° 366, p. 1 20. 



Wednesday, June 1, 1720, being at anchor near Revel, in 58° 58' north 

 lat. the magnetical amplitude at sun-set was west, 64° 30' north, and the true 

 amplitude was west, 49° 37' north; therefore the variation north, 14" 53' west. 



Saturday, July 23, at the isle Gottsand, in 58° 2l' north lat. at sun-set, the 

 magnetical amplitude was west, 49° 50' north, and the true amplitude west, 

 35° north, which gives the variation north, 14° 50' west. 



The difference of longitudes of the two foresaid places by the dead reckoning 

 is 1° 50'. 



October the 24th at Bornholme, in 56° lat. at sun-rising, the magnetical 

 amplitude was east, 43° 15' south, and the true was east, 28° 31', which gives 

 the variation north, 14° 44' west. 



