April r, 1875] 



NATURE 



431 



about forty in the great Pipe-lish (.Syn^natlius aciis). In 

 illustration of the amount of force expended in the 

 ivorking of its propeller, it may hi mentioned that Prof. 

 Lankeslcr finds that it is only in the above-described 

 muscles, by which it is move:!, and in no other part of the 

 body, that the red- colouring haemoglobin is to be de- 

 tected. 



THE NEW STANDARD SIDEREAL CLOCK OF 

 THE ROYAL OBSERVATORY, GREENWICH 



T^HE Royal Observatory at Greenwich has lately ac- 

 -'■ quired a new standard sidereal clock which pos- 

 sesses several peculiarities of construction. The one 

 formerly in use was that made by Hardy, and originally 



Fiff.l. 



fitted with Hardy's escapement, although this had many 

 years ago been removed and an ordmary dead beat 

 escapement substituted. This clock was a celebrated 

 one in its day, but of late years it seemed scarcely to 

 satisfy modern requirements, and it was decided that a 

 new one should be constructed. This has now been 

 done. The new clock was planned generally by the Astro- 

 nomer Royal, and constructed entirely by Messrs. 

 E. Dent and Co., of the Strand. It was completed 

 and brought into use in the year 1S71, and both as 

 regards quality of workmanship and accuracy of per- 

 formance it appears to be an excellent specimen of 



horological art. As in the galvanic system of registration 

 of transit observations it is unnecessary that the clock 

 should be within hearing or view of the observer, the new 

 clock has been fixed in the Magnetic Basement, 'in which 

 the temperature varies only a very few degrees durino- 

 the course of a year. 



The pendulum is supported by a large and solid brass 

 casting securely fixed to the wall of the basement, and 

 the clock movement is carried by a platform forming 

 pirt of the same casting. The Astronomer Royal adopted 

 a form of escapement analogous to the detached chrono- 

 meter escapement, one that he had himself many years 

 before proposed for use,* in which the pendulum is free, 

 excepting at the time of unlocking the wheel and receiving 

 the impulse. Several clocks having half-seconds pen° 

 dulum had since been rnade with escapement of this kind, 

 but the principle had not before been applied to a large 

 clock. The details of the escapement may be seen in 

 Fig. I, which gives a general view of a portion of the 

 b.ick plate of the clock movement, supposing the pendu- 

 lum removed : a and b arc the front and back plates 

 respectively of the clock train ; i- is a cock supporting one 

 end of the cru'ch axis ; (/ is the cru'.ch rod carrying the 



pallets, and c an arm cirried by the crutch axis and fi.xtd 

 aty to the left-hand pallet arm ; ^ is a cock supporting a 

 detent projecting towards the left and curved at its 

 extreme end ; at a point near the top of the escape wheel 

 this detent carries a pin (jewel) for locking the wheel, 

 and at its extreme end there is a very light " passing 

 spring." The action of the escapement is as follows : — 

 Suppose the pendulum to be swinging from the right 

 hand. It swings quite freely until a pin at the end of the 

 arm e lifts the detent ; the wheel escapes from the jewel 

 before mentioned, and the tooth next above the left-hand 

 pallet drops on the face of the pallet (,the state shown in 

 the figure) and gives impulse to the pendulum ; the wheel 

 is immediately locked again by the jewel, and the pen- 



* la the year 1827. ia a paper " On the Disturbances of Pendulums and 

 lialances, and on the Theory of Escapements/' which appears in the third 

 volume of the Transactions 0/ the Camiri'/S' FhilasofhUal Stcicty, 



