26 PROCEEDINGS OF THE AMERICAN ACADEMY. 



above the center of the grandstand, and terminating at a point just 

 beyond the boulevard recently built by the Metropolitan Park Com- 

 mission. The north and south direction was chosen in order that the 

 observations might not be interfered with by the sun at the times of 

 day most favorable for flight. It was expected that the aviators would 

 cross this line near its center on each coil of the ascending spiral, and 

 that the greatest height attained at such a crossing would be taken as 

 the height of record. 



At each end of the field base was placed a sextant mounted on a 

 stand (Plate 2), with its plane in the vertical plane of the base line, 

 and the line of sight pointed toward the grandstand, which was nearly 

 midway between the stations. A target placed in the line of sight at 

 a sufficient distance from each sextant served as a substitute for the 

 horizon line, and when the image of the aeroplane was brought to coin- 

 cide with the target, as it crossed the plane of reference, the altitude 

 was read directly from the vernier of the sextant. The height of the 

 northern station was eighteen feet and of the southern station twenty- 

 six feet above mean low water ; it is sufficiently accurate to assume 

 that the sextants were in the same horizontal plane twenty-two feet 

 above mean low water and nine feet above the center of the field. 



After some experiments the target was given the form shown in 

 Plate 2. The central lozenge was one foot square and the extreme 

 horizontal length four feet. The wings served to make an approximate 

 setting as the aeroplane approached the plane from either side. The 

 horizon mirror of the sextant was replaced by a larger mirror extending 

 about three inches from the sextant plane, the silvering being removed 

 from a strip one eighth of an inch wide perpendicular to the plane of the 

 sextant. Through this opening the target could be seen by the naked 

 eye placed somewhat to the left of the telescope, and the aeroplane 

 could thus be picked up and followed and an approximate setting 

 made some time before coming to the reference plane. At the critical 

 moment the eye was placed at the telescope and the image of the aero- 

 plane made to cross the center of the lozenge of the target. The use 

 of a light blue shade between the index and horizon glass greatly facil- 

 itated the observation by cutting off the glare of the sky. The clamp 

 of the index was replaced by a roller turned by a good-sized milled 

 head, and forming a very convenient " quick slow motion." A "finding 

 plane " of a size corresponding to the new horizon glass was attached 

 to the index glass and carried a level perpendicular to the plane of 

 the sextant, by means of which that plane might be made vertical. 

 These three modifications are all plainly shown in Plate 2. 



The observers were connected with each other and with the commit- 



