OF ARTS AND SCIENCES: JANUARY 9, 1866. 65 



and not to repeated reflections from the surface of drops of water, as 

 some physicists have supposed. 



As can readily be seen, the aqueous lines of the solar spectrum 

 present a very wide field for investigation, but one which can only be 

 cultivated under peculiar atmospheric conditions. This paper is only 

 intended to open the subject. I hope to be able to continue the study 

 on every favorable opportunity, and shall take pleasure in communicat- 

 ing any future results to this Academy. 



Professor .Charles W. Eliot exhibited to the Academy a dy- 

 namometer invented by Mr. S. P. Ruggles, the Curator of the 

 Museum of the Institute of Technology. 



" This new and admirable invention accomplishes two objects ; — 

 first, it measures the exact amqunt of power which is being consumed 

 in driving a single machine, or any number of machines, at any instant 

 of time, indicating every change in the force required, as the work done 

 by the machines varies from instant to instant ; secondly, the apparatus 

 adds up and registers the total amount of power which has been used 

 by any machine, or set of machines, during a day, a week, a month 

 or any desired time. The apparatus may be thus described. The 

 pulley, from which the power is taken, is attached to the shaft by the 

 intervention of a spiral spring. One end of this spring is secured to 

 the shaft, and the other end to the hub of the pulley. The lateral mo- 

 tion of the pulley upon the shaft is prevented by a collar on either side 

 of the pulley. On the inside of the hub is cut a screw of about three- 

 inch pitch, that is, a screw which makes a complete turn within a dis- 

 tance of about three inches measured on the axis of the hub. A rec- 

 tangular slot is cut out of that part of the shaft which lies within the 

 hub of the pulley, and in this slot slips backwards or forwards a piece of 

 metal which precisely fits the slot. From each side of this small piece 

 of metal there projects beyond the surface of the shaft a small portion 

 of the male screw which exactly fits into the screw cut in the interior 

 of the hub of the pulley. If there be no resistance at all to the mo- 

 tion of the pulley, the shaft, spring, and pulley will all start together, and 

 revolve together. But if a resistance be offered to the motion of the 

 pulley, the shaft, and with it the piece of metal which slips in the slot, 

 will start first, and the pulley will move only when the strain caused by 

 the twisting of the spring is sufficient to overcome the resistance ap- 

 plied to the circumference of the pulley. But if the piece of metal in 

 VOL. VII. 9 



