270 PROCEEDINGS OF THE AMERICAN ACADEMY 



axis perpendicular to its own axis, gives both constants together, with- 

 out distinguishing between them. The direction of the three axes 

 around which the telescope should be rotated to give the coUimation, 

 level, or both combined, may be remembered as the axis of the tele- 

 scope, the vertical cross-hair, and the horizontal cross-hair, respectively. 

 With this modification, the micrometer level compares favorably with 

 the vertical circle as regards economy in time and expense, portability, 

 and accuracy when small angles are to be measured. 



Before this modification, the difiiculty in determining the combined 

 level and collimation error was avoided by observing the difference in 

 height of a distant and near mountain which are nearly in line. These 

 were commonly so selected as to be in the same field of view. It was 

 therefore only necessary to observe the difference in altitude by the 

 micrometer screw under various atraosijheric conditions. 



The first series of measurements were made on Mt. Wachusett, in 

 Princeton, Mass., on June 22, 1877. Accompanied by Col. C. W. 

 Folsom, I reached the summit at about noon. The observations were 

 begun at once with instrument B, and wei'e continued with short inter- 

 missions until nearly eight o'clock, when darkness interfered. The 

 following morning was exceptionally clear, and observations were be- 

 gun at a quarter after four, and continued until two in the afternoon, 

 when we left the summit. 



Mt. Monadnock, Cheshn-e Co., N. H., was visited, July 3, 1877, by 

 Mr. J. R. Edmands, Professor C. E. Fay, and myself. We reached 

 the summit at about three in the afternoon, and began observing with 

 instrument B. The air was so hazy that the Sandwich range was 

 barely visible, and Ascutney, the Southern Kearsarge, and Gunstock 

 were the most distant points observed. The observations were aban- 

 doned at about five o'clock on account of the increasing haze. On 

 July 15, 1877, Mr. Edmands and I visited Mt. Washington, in the 

 White Mountains, where we remained until July 24. The almost con- 

 tinuous cloud and haze prevented much work, except on July 18, when 

 a few preliminary observations were made with instrument C: and on 

 July 21 and 22, when observations were obtained with D. Mt. Kear- 

 sarge, Merrimack Co., N. H., was occupied at the same time by Mr. 

 J. B. Henck, Jr., with instrument A, in order that simultaneous ob- 

 servations might be obtained at the ends of the line from Mt. Wash- 

 ington to Kearsarge. 



Observations were undertaken in August and September, 1877, 

 at Jefferson Hill, N. II., by Mr. Henck, and at East Jefferson, by 

 Mr. Edmands, with instruments B and D, respectively. Again long 



