316 Dr. Mac Cu l l o c h on the Parallel Roads of Glen Roy. 



justify. The necessity of investigating their probable or possible 

 origin from natural causes will, in consequence of the extent of 

 their geographical connections, lead to rather a wide range of 

 enquiry, not however wider than the importance of the subject 

 will be found to justify. I have attempted to keep clear of all spe- 

 culations purely hypothetical, and wherever physical evidence and 

 analogy have entirely failed, have rather chosen to leave the ques- 

 tion in its natural obscurity than to involve it in more profound 

 darkness by assigning imaginary causes. To avoid any bias which 

 the original and not sufficiently descriptive term, Parallel roads, 

 might preserve in the reader's mind, I have substituted that of 

 Lines, a term less exceptionable and sufficiently expressive of their 

 appearance.* 



* I think it right to remark that every precaution was taken in ascertaining both 

 the levels and the elevations which will be referred to in the arguments hereafter to be 

 used. Those which could not be accurately determined, on account of the nature and 

 extent of the ground requiring a survey, have been estimated by such approximations as 

 were attainable, and these are distinguished wherever they occur. However imperfect, 

 they are no where so lax as to affect the arguments, even if the errors were much greater 

 than any which could have occurred, as no undue stress is any where laid on hypothetical 

 assumptions. The levels of the lines were observed by a spirit level, and the vertical 

 distances between them were measured by the same instrument. One barometer of 

 Ramsden's construction was applied for the measurement of the elevations; and such 

 differences between any two altitudes as were required, were ascertained by observations 

 repeated at very short intervaJs. For determining the absolute altitudes above the level 

 of the sea, the barometric observations werfl compared with a register kept purposely for 

 these experiments by Lord Gray at Kinfauns Castle, those observations, with the required 

 corrections, being made at the same hours with instruments of similar construction. To 

 remove still more any chance of error, the principal altitudes were deduced from a medium 

 of nine observations taken on four different days, the greatest variations between the ex- 

 tremes not exceeding one twelfth of the whole. The altitude of the great Caledonian 

 Talley is known from the measurements belonging to the canal. 



Notwithstanding all this care however, I can only consider these altitudes as approx- 

 imations, since I am convinced from a careful comparison of barometric registers that this 

 instrument cannot be relied on when used in this maimer, as I have attempted to show in. 

 a late communication to this Society. 



