TRANSACTIONS OF THE SECTIONS. 49 



On a Modification of the Sprengel Pump, and a new Form of Vacuum-Tap, 



By 0. H. GiMiNSHAM. 



On iiew Standards of Measure and Weight, By Prof. IJennessy, F.E.S. 



On a new Form of Thermojneter for ohservinrj Earth Temperature, 



By G. J. Symons. 



On an UnmistaTcahle True North Compass. By G. J. Ststons. 



The aiitlior said that it was not generally known, except to nautical and to 

 scientific men, that the compasses usually sold did not point to the true North or 

 South Pole of the earth. The magnetic Pole, to which all compass-needles pointed, 

 was not identical with the geographical pole, which was the north point of 

 maps. The variation of the needle was considerable, and was no doubt often 

 the cause of tourists losing their way. The difference between true and mag^ietic 

 north was not the same in all parts of the United Kingdom, and a fortiori in all 

 parts of the globe, nor was it absolutely the same from year to year. One of the 

 advantages of these instruments was their pointing to the true north, the other 

 was their " unmistakableness." These compasses were corrected for use in the United 

 Kingdom, but could be adapted to any specified locality in any part of the world. 



0)1 a new Form of Astronomical Clock with Free Pendulum and Indepen- 

 dently Governed Uniform Motion for Escapement-iuheel, By Sir "W. 

 I^ou^o^, D.C.L., F.B.S. 



The object of this communication was to explain to members of the Association 

 and give them an opportunity of seeing in the authors house in the_ University a 

 clock which had been described in a communication to the Royal Society, in 1869, 

 entitled " On a New Asti'onomical Clock and a Pendulum Governor for Uniform 

 Motion." The following description is taken from the ' Proceedings of the Royal 

 Society ' for 1869, except a few alterations and additions and the drawings, which 

 have not been hitherto published : — 



It seems strange that the dead-beat escapement should still hold its place in the 

 astronomical clock, when its geometrical transformation, the cylinder escapement 

 of the same inventor, Graham, only survives in Geneva watches of the cheaper class. 

 For better portable time-keepers it has been altered through the vicious rack-and- 

 pinion movement into the superlatively good detached lever. If it is possible to 

 make astronomical clocks go better than at present by merely giving them a better 

 escapement, it is quite certain that one on the same principle as the detached lever, 

 or as Earnshaw's ship-chronometer escapement, would improve their time-keeping. 



But the irregularities hitherto tolerated in astronomical clocks may be due more 

 to the faultiness of the steel and mercury compensation pendulum, with its loosely 

 attached glass jar, and of the mode in which it is hung, and of the instability of 

 the supporting clock-case or framework, than to imperfection of the escapement 

 and the greatness of the arc of vibration which it requires ; therefore it would be 

 wrong to expect confidently much improvement in the time-keeping merely from 

 improvement of the escapement. I have therefore endeavoured to improve both 

 the compensation for change of temperature in the pendulum, and the mode of its 

 support, in a clock which I have recently made with an escapement on a new prin- 

 ciple, in which the simplicity of the dead-beat escapement of Graham is retained, 

 while its great defect, the stopping of the whole train of wheels by pressure of a 

 tooth upon a surface moving with the pendulum, is remedied. 



Imagine the escapement-wheel of a common dead-beat clock to be mounted on a 

 collar fitting easily upon a shaft, instead of being rigidly attached to it. Let friction 

 be properly applied between the shaft and the collar, so that the wheel shall be 

 carried round by the shaft unless resisted by a force exceeding some small definite 



