1892.] 235 [Bache. 



scribed, on the respective horizons as plotted on the paper to 

 become a map. 



Balloon photogrammetry has been practiced to some extent ever 

 since the invention of the photographic dry plate. This method, 

 however, has belonged rather to the sphere of reconnoissance than 

 to that of surveying. When some prominent objects appear on the 

 landscape, whose geographical relations to one another are known, 

 the balloon photographic product may be of considerable value, if 

 too large a circle has not been included by the camera ; and this 

 method indicated, if the desirable conditions are strictly fulfilled, 

 may be utilized to advantage if the resulting map is not required to 

 be of rigid accuracy. When, however, such objects are very remote 

 from one another, even when their geographical positions are 

 known, the spherical aberration resulting from employing a large 

 aperture of object-glass makes a product which cannot be regarded 

 as of high value, one which cannot properly be dignified with, the 

 name of survey in the restricted sense of the term, and to which 

 we should prefer to apply the name of reconnoissance. Without 

 adjusted height for the camera, without near objects of known 

 geographical relations to one another, to obtain orientation for the 

 results, without precise regulation of the angular aperture of the 

 object-glass of the camera, nothing can be produced by balloon 

 photographic process that, in the restricted sense noted, merits the 

 name of survey. 



It is on account of my perception of this low estate of balloon 

 photogram.metry that my attention is especially drawn to devising 

 a method of applying the art upon true principles. By my method 

 the balloon must be captive, not free, and being captive it may be 

 made quite small, easily managed, and inexpensive, thus rendering 

 its employment practicable for ordinary use, especially as, accord- 

 ing to the plan sometimes adopted in the case of the military cap- 

 tive balloon (to the consideration of which we shall presently 

 come), the gas requisite for inflating the balloon can readily be 

 carried under high pressure in metallic cylinders. 



The traverse line of land surveying is merely a zigzag course, 

 consisting of stations, the angle between each successive three of 

 which, and distance between each successive two, is measured. 

 From these stations details of the terrene are generally procured. 



