137 



The American Journal of the Medical Sciences. Edited by Isaac- 

 Hays, M.D. No. 5. New Series. Philadelphia, 1842.— From 

 the Editor. 



The Committee, consisting of Mr. Lea, Mr. Taylor, and 

 Mr. Vanuxem, to whom was referred the paper of Professors 

 William B. Rogers and Henry D. Rogers, entitled " Observa- 

 tions on the Geology of the Western Peninsula of Upper 

 Canada, and the Western part of Ohio," reported in favour of 

 its publication in the Society's Transactions, which was ordered 

 accordingly. 



Major Bache made an oral communication in relation to a 

 method which he had recently practised for obtaining the mag- 

 netic meridian. 



He stated that the instrument used in the operation was a theodo- 

 lite of the ordinary English construction, having eight inch plates, 

 with double verniers, reading to fifteen seconds, and a needle, 3 J 

 inches in length, moving in a compass box graduated to degrees. 

 The method consists in measuring two equal arcs with the needle, 

 one on each side of the zero, and referring them to the graduation on 

 the limb of the instrument ; the line bisecting the sum of these, as 

 shown by the same means, being assumed as the magnetic meridian. 

 The advantages which this mode has over the ordinary one by a 

 single reading of the needle, consist in the means afforded by the 

 graduation on the limb of the instrument, for ascertaining the proba- 

 ble error in each single determination of the meridian ; and for refer- 

 ring any number of sets of observations to a fixed graduated plate, 

 by which a mean of a series may be obtained. The mode of opera- 

 ting is as follows : the theodolite being levelled, and the horizontal 

 limb undamped, the upper plate is moved by hand until the needle 

 coincides nearly with the graduation at 45° on the compass box. The 

 limb is then clamped, and the final movement made with the tangent 

 screw; the observer placing himself for that purpose in range with 

 the centre pin and given line on the compass box ; the coincidence 

 being considered satisfactory when the eye can no longer distinguish 

 the end of the needle from the end of the line of the graduation. The 

 reading on the limb is then made. The plates are now undamped, 

 and the upper one turned to bring the needle, as in the first instance, 

 near the graduation at 45° on the other side of the zero; and the ope- 

 ration for obtaining a nice coincidence is performed, and the limb a 



