56 On the Use of the Pocket Box-Sextant. 



rately, is of some consequence to a military man or traveller ; but 

 there are many other useful purposes to which it can be applied : a 

 brief examination of them is sufficient in the compass of a short 

 essay, the chief object of which is to draw the attention of travellers 

 to an instrument not so well known or appreciated as it deserves 

 to be, the merits of which were ascertained by many experiments 

 made expressly with a view to determine its powers, and the facts 

 are left to speak for themselves. 



For altitudes accessible or not, when the ground is level, very 

 near results may be obtained by means of a small table engraven 

 on the cover : it is simply a table of natural tangents expressed as 

 multipliers or divisions of the radix, and is very useful in finding 

 the heights of churches, columns, or other buildings on level 

 ground ; for, with the sextant alone, and without any other tables 

 than this, a base being paced or otherwise measured by a walking- 

 cane, or whatever may be at hand on the occasion, the altitude of 

 such objects may be found within a few inches of the truth. 



We will now suppose a traveller about to make a plan or sketch 

 of the site of some ancient town, or any interesting place, and that 

 he has no other instrument than the sextant : we have seen that for 

 lines of 12 inches in length, on paper, it can be depended upon, and 

 the practice of those who use it, is to determine as many points in 

 this manner as may seem necessary, sketching the intermediate 

 objects by the eye : much is certainly left to the eye, but uncer- 

 tainty may be diminished by determining more points, and thus 

 any assignable degree of correctness will follow. Many hundreds 

 of square miles have been sketched in this manner during the last 

 war, and it has been found to answer extremely well : the military 

 officer or other traveller is thus relieved of the burden of more 

 cumbrous apparatus; a small telescope to find his distant object 

 with more certainty is all the additional assistance required in 

 the field, when his base has been measured, and while the operation 

 of fixing points is going on ; a paper containing those points is all 

 he requires in the sketching : his sabretashe holds this paper; and 

 a protractor, with a few drawing instruments at home, completes 



