106 proceedings: philosophical society 



on which the instrument rests and an outer four-legged structure called 

 the scaffold which supports the observer and the lightkeeper. The 

 two structures are entirely independent of each other and do not touch 

 at any point. The reel showed the operation of erecting one of these 

 towers from the time the building party arrived at the station until 

 the structure was entirely completed. These towers are usually from 

 40 to 60 feet in height and are built of lumber purchased at the point 

 most convenient to the field of operation. The legs of the structure 

 are set well into the ground and are strongly anchored to prevent their 

 being blown over by the wind. 



The third reel showed the method of conducting the observing party. 

 It showed the automobile trucks used in carrying the party and outfit 

 from station to station and the various operations connected with the 

 observing. It also showed the lightkeeper at work with his signal 

 lamp or heliograph. Practically all of the observations of primary 

 triangulation are now made on sunlight reflected from a mirror called 

 a heliograph and on the signal lamps used at night. The modern 

 signal lamp has a very high candlepower and the light can be sent 

 over long lines and through atmosphere which would make observing 

 impossible with the old style of lamp. The new electric lamp is one 

 having contracted filament with dry cells furnishing the electric cur- 

 rent. In the older type of lamp the fuel used was acetylene gas. 



The primary triangulation is carried on much more rapidly today 

 than ever before, but there has been no decrease in the accuracy of the 

 observations. The accuracy of the triangulation is that expressed by 

 an average closing error of about 1". The probable error of any 

 one direction in the triangulation is usuall}^ less than Y- This 

 accuracy is very great, as is shown by the fact that one foot at a dis- 

 tance of 40 miles from the observer subtends an angle of 1". The 

 accuracy in the results is obtained by repeating the angles a number 

 of times and taking the mean. The errors are probably caused by 

 atmospheric conditions. 



H. L. Curtis, Recording Secretary. 



