Oct. 26, 1876] 



NATURE 



575 



and liberate the wheel ; the cog l\ will then immediately 

 operate upon the hook H^ and hft the other pallet. Mean- 

 while the pendu'um swings away to the right, carrying 

 the pallet P2P2, and returns with it, but as there is now no 

 cog P2 to receive it, it falls to the lower position corre- 

 sponding to that now occupied by Pj P^, the excess of 

 its fall over its rise, upon the pendulum as in the pre- 

 ceding case constituting the impulse. 



This form of gravity escapement has been further 

 modified and improved for ordinary use by Sir Edmund 

 Beckett.^ The only way in which variations in the 

 force of the clock-train can disturb the pendulum in 

 these escapements is by putting more or less pressure 

 upon the locking studs, giving the pendulum more or 

 less trouble in liberatmg the escapement; and with refer- 

 ence to this you must not be deceived by so-called im- 

 provements for detaching the pendulum completely from 

 the escapement, for they really never do so, and generally 

 by the number of pieces employed, hamper the pendulum 

 with much more friction than that to which it would be 

 exposed by direct communication with the clock-train. 



You will see that the general effect of a gravity escape- 

 ment is to make the pendulum move rather faster than if 

 it were a free one, because the weight of the pallets is 

 equivalent to two smaller pendulums attached to it during 

 the greater portion of its swing. And the effect of any 

 increase of pressure is quite the reverse of what would 

 happen with a direct escapement, for it increases the 

 pressure upon the lockings without increasing the im- 

 pulse, and will consequently cause the are of vibration to 

 fall off. 



The last clock-escapement I shall describe is a de- 

 tached one (see Fig. 18), the design of the Astronomer 

 Royal, Sir George Airy. 



There is only one pallet, A, the other arm, B, being 

 merely a safety-catch and counterpoise. That tooth of 

 the escape-wheel, C, is not really resting upon the dead 

 face of the pallet, though it is very close to it, the wheel 

 being at present held by the detent, D, fastened to the 

 clock frame. 



The pendulum is supposed to have reached the 

 limit of its excursion towards the left, and to be now 

 returning. When it reaches a certain angle before zero, 

 a pin, H, in the arm K (which swings with the pen- 

 dulum and pallets), passes under the detent, lifts it, and 

 unlocks the wheel at just that instant that the tooth c 

 shall fall immediately upon the impulse face of the 

 pallet without touching the dead face at all. The tooth 

 slides down the impulse face, giving impulse to the 

 pendulum ; meanwhile, the pin H passes on and allows 

 the detent to fall in time to catch the succeeding tooth 

 L. The tooth quits the impulse face when the pen- 

 dulum is at the same angle after ztro that it was at 

 before zero when the impulse began. Thus you get an 

 equal impulse when the pendulum is falling as when it is 

 rising, the advantage of which I pointed out to you when 

 we were discussing Graham's dead escapement. Besides 

 this, you get no dead friction, and the pendulum is almost 

 completely detached from the clock-train. Upon return- 

 ing the pin H clears the detent this way. You see that 

 long sprmg beneath the detent, commencing near its 

 middle, and projecting beyond its extremity upon the 

 right ; just now, in unlocking, the extremity of the 

 detent supported this spring, and detent and all gave way 

 before the pin H. But upon returning, the extremity of 

 the detent of course gives no support to the spring, and 

 the pin H pushes it upon one side without disturbing the 

 detent. This escapement is used in the normal sidereal 

 clock at Greenwich. 



[To be continued.') 



• The Wes minster Clock has one of his forms. A locking stud is placed 

 upon the back of one pallet and the front of the other, ana there are two 

 collections of arms (ot th)eeeach)on euher side of the cog-wheel, to meet 

 them. The cog-wheel itself has also three cogs. This escape- wheel, with 

 a stfconds pendulum, turns once in six seconds, and its velocity U controlled 

 by a fly. 



CHARLES SAINTE-CLAIRE DEVILLE 

 ]Y| CHARLES SAINTE-CLAIRE DEVILLE, the 

 •l-Vl* distinguished geologist and meteorologist, and 

 brother of M. Henri Sainte- Claire Deville, the well-known 

 chemist, was born of French parents in 18 14, at St. 

 Thomas, in the West Indies. At the age of 19 he was 

 enrolled a pupil of the School of Mines, in Paris, and 

 after a course of study there undertook, at his own expense, 

 a scientific expedition extending from 1839 to 1843, to 

 the Antilles, Teneriffe, and Cape Verd Islands. He spent 

 upwards of a year investigating the geology of Guada- 

 loupe, and wrote a detailed account of the terrible earth- 

 quake which laid waste that island in 1843. The results 

 of this expedition he published in two series of memoirs:, 

 the one appearing from 1856 to 1864, on the geology of the 

 Antilles, Teneriffe, and Cape Verd Islands, and the other 

 from 1 86 1 to 1864, principally on the meteorology of the 

 Antilles. He was sent by the Institute to Italy in 1855 to 

 examine the great eruption of Vesuvius which occurred in 

 that year. After attentively following and investigating the 

 eruption through all its phases, he wrote a description of 

 it in a series of letters addressed to M. Elie de Beau- 

 mont, which were published in the Comptes Rendus and the 

 Moniteur during 1856. He also, in 1858, published an 

 interesting account of the volcanic eruptions of Stromboli, 

 in the Lipari Isles, and in later years, various papers on 

 other volcanic eruptions. Several memoirs on different 

 points in chemistry and physics were written by him 

 about 1852, and for several years he filled with distinction 

 the chair of geology in the College of France, formerly 

 held by the illustrious Elie de Beaumont. On December 

 28, 1857, he was elected a member of the French Academy 

 of Sciences in the place of Dufrenoy, and on August 

 13, 1862, was made an officer of the Legion of Honour. 



During the time he worked in the laboratory of his 

 friend M. Dumas, he discovered the amorphous and in- 

 soluble form of sulphur, thus poirUing out for the first 

 time the fact that an elementary body may at will be 

 made to assume two totally distinct states, differing from 

 each other not only as regards their physical characters, 

 but also as regards their essential chemical properties. 

 This discovery was published in 1852. 



Shortly after this his attention began to be more 

 decidedly attracted towards meteorology ; so much so, 

 indeed, that for the past twelve years he appears in his 

 writings almost exclusively as a meteorologist. Indeed 

 the meteorological work, both scientific and administra- 

 tive, which he undertook to do, and which he did, was so 

 laborious and harassing as to leave him little time for 

 other pursuits. By this work, however, he has left his 

 mark unrnistakably on the meteorology of France. 



The fruits of his meteorological researches were given 

 to the world in a remarkable series of papers in the 

 Comptes Rendus during 1865-67, on the " Periodic Variations 

 of Temperature." The object of this investigation was 

 to prove the existence of annual and super-annual periodic 

 perturbations of temperature, and to state with precision 

 the character and nature of these period?. Having shown 

 the occurrence of similar perturbations of temperature on 

 four days of the same date in February, May, August, 

 and November, these days being placed on the terrestrial 

 orbit at equal intervals, and which, by the way, correspond 

 with the dates of the festivals of the "Ice Saints," he 

 inquired how far similar perturbations occur on any 

 four days of the year separated from each other by 

 equal intervals of time. Since the observations showed 

 that some years and groups of years presented for 

 the same days perturbations different from those of 

 other years, being sometimes above and sometimes 

 below the normal means of the days, an inquiry was 

 raised as to the Hmits of the antagonism thus disclosed 

 both as regards the amount and the cycle of years 

 it embraced. Lastly, since these perturbations, if they 

 exist, must exercise an important influence on all the 



