30 PROCEEDINGS OF THE AMERICAN ACADEMY 



The water flowed toward the north. Distances indicating position 

 of orifices were measured when the trough was level. 



The trough with its appurtenances heing in place, the whole was 

 covered with a house about eight feet wide and ten feet high, having 

 windows at the top. 



The comparative heights of the water surface in the stream and in 

 the reservoirs adjacent were now to be determined. The first step 

 was to measure the heights of three points at each station — one over 

 the middle of the stream, and one over each of the still-boxes — above 

 a surface of still water. This was done by the aid of three kinds of 

 instruments. The first kind, by which any change in height of the 

 water surface was noted, consisted of a plate of brass placed horizon- 

 tally, through which projected vertically upward fifty-one steel nee- 

 dles in two rows. The first needle being finished with its point just 

 0.1 ft. above the bottom of the plate, and the fifty-first having its point 

 0.11 ft. above the same surface, the intermediate points being sepa- 

 rated by equal spaces, were finished to be in the same inclined plane 

 with the extremities ; hence each point was 0.0002 ft. higher than the 

 next lower. 



Six other needles, rising above these in another row, indicated the 

 position of the points reading two-thousandths. 



The second kind of instrument consisted of a rod having a scale 

 divided into hundredths of a foot, sliding along a short standard hav- 

 ing a stationary vernier reading to thousandths, by which distances of 

 two ten-thousandths of a foot could be readily distinguished. The rods 

 were held in a vertical position by fitting into frames above each point 

 of observation. They were terminated at the lower end by a long finely 

 pointed needle, which was brought in contact with the water surface. 



The third kind of instrument consisted of a vertical micrometer 

 screw, piercing a horizontal iron plate which made a part of its nut, 

 and whose under surface was kept level by a level bulb upon its upper 

 surface. The screw was terminated below with a finely pointed nee- 

 dle, and above, near the head, was supplied with an index, whose 

 position was read upon a circular scale made upon the top of the nut, 

 in which one ten-thousandth of a foot was indicated by the space of 

 about one one-hundredth of a foot. 



After determining by these instruments the actual heights above a 

 datum plane of all of the points where observations were to be taken, 

 the same instruments were used upon the same points for determining 

 the heights of the water surfaces, when water was flowing through 

 the trough. 



