1892.1 ^^* [Bache. 



angles and directions of the parts of the line successively submitted 

 to its operation, as well as of delineating what it must perforce in- 

 clude by the photographic process in the representation of the 

 details of the subjacent terrene. In this method the end link of a 

 given section of the line would have to be duplicated in the 

 advancing survey of the line, so that the relations with one another 

 of all parts of the line should be maintained. If, additionally, the 

 azimuth of one of the links of the line were obtained, it would 

 communicate azimuth to all the other links. But this method can, 

 at best, be recommended for nothing beyond the requirements of 

 reconnoissance. 



The photographing of a link of a traverse line in the precise 

 manner first described involves, of course, the necessity that the 

 balloon and each of the two stations representing the link over the 

 middle of which it is floating, should be intervisible. A similar 

 condition, as between the two stations as viewed on the ground, is 

 indispensable. It is evident, however, that if there are trees or 

 other obstructions on the ground, the stations might be intervisible 

 below, and yet that each might not be intervisible with the balloon. 

 Consequently, as not only these conditions but the condition of 

 ample space for the management of the guys must be fulfilled, pre- 

 cise operations with the balloon imply the existence of open 

 ground, or ground substantially free from obstructions to sight. 



In proportion as the balloon is allowed to attain a greater and 

 greater height, so as to include more and more of the earth's sur- 

 face, the scale of the resultant map would become smaller and 

 smaller, and the apparatus more and more unmanageable, because, 

 at a great height the guys cannot be maintained at the angles 

 requisite to control its exact position. Therefore, it will in practice 

 probably be found that heights of from three hundred and fifty 

 to five hundred feet will be those most convenient for surveying by 

 this method. 



One gain made by elevation is more than counterbalanced by the 

 loss of the clearness of delineation that belongs to a large scale. 

 It is evident that, at moderate heights, the photographic projection 

 of an abrupt rise of ground or other object, as, for instance, a 

 house, on the plane of the photograph is at a greater distance by 

 scale from the vertical passing through the balloon than it should 

 be as related to nature, but that, as the height of the balloon above, 

 the earth increases, this error proportionately decreases. There- 



