Intelligence and Miscellaneous Articles. 395 



I describe in detail in the memoir the precautions to be taken in 

 order to adjust in the same right line the optic axes of the two ap- 

 paratus, and to place the surface of the mirror in the focal plane of 

 the collimator. This second condition must be fulfilled very accu- 

 rately; otherwise the loss of light on the return would be consider- 

 able. I succeeded in fulfilling it completely by using as a reticule 

 the silver pellicle partially raised on certain points of the mirror : 

 the precision is then determined entirely by the defining-power of 

 the telescope. 



Among the improvements introduced in the course of these re- 

 searches, I will mention the construction of the motor of the toothed 

 wheel. Froment's motor, with helicoidal tooth-range, was given up, 

 as it necessitated too great a motive force. I notably simplified the 

 arrangement by utilizing some clockwork mechanisms sold under 

 the name of roulants carves (the sides from 12 to 15 centims.) ; the 

 escapement and wheelwork are taken away, and the ratchet-wheel 

 of the escapement is replaced by a lighter wheel with finer teeth : 

 I used for this purpose three patterns, wheels with 104, 116, and 

 140 teeth. 



By fitting a powerful spring in the barrel, I have been able to 

 attain velocities of from 700 to 800 turns in a second. Finally, as 

 a complement, on the axle of the minute-hand I disposed an electric 

 cam (necessary for the registration of the velocity of rotation of the 

 mechanism), a brake (to regulate the velocity at will), and a second 

 barrel (permitting the toothed wheel to be turned in the contrary 

 direction). This last arrangement is useful for the elimination of 

 certain systematic errors which might result from the mechanism 

 itself. 



I shall not dwell upon the description of an experiment. The 

 observer, attentive to the variations of intensity of the return-light, 

 transmits electric signals to the recorder — that is, a cylinder covered 

 with blackened paper, on which three electromagnets cause to be 

 traced respectively the signals from the seconds-clock, from the cam 

 of the mechanism, and from the key managed by the observer. In 

 general he notes the successive disappearances of the light, which 

 correspond to velocities of the toothed wheel, varying as the series 

 of the odd numbers. Thanks to the working of the brake, he can 

 at will produce an acceleration or retardation of the motion of the 

 mechanism, or maintain a velocity sensibly constant during some 

 seconds. 



Notwithstanding the disadvantages of the atmosphere of Paris, I 

 often obtained a very intense return-light with the oxyhydrogen 

 lamp, or even with a simple petroleum-lamp. The total number of 

 my observations exceeds a thousand; they are registered in the 

 form of graphic traces, which I have the honour to lay before the 

 Academy. The work of reduction is rather tedious; so I have 

 taken up only those observations which are most complete and were 

 made under satisfactory circumstances ; their number amounts to 

 about 650. A uniform method of calculation permitted me to de- 

 duce from the traces the time which the fight occupied in accom- 



