61 LECTURES TO SCIENCE TEACHERS. 



edge, and with its plane surface normal to the axis of the 

 beam. The observer is placed about 12 feet distant from the 

 mirror, and views through a telescope the reflection in the 

 mirror of a vertical graduated scale placed close to the tele- 

 scope. The ray of light reflected from the mirror and thrown 

 upon the graduated scale serves to indicate with great 

 exactness by its angular deviation the difference of the 

 standard weights compared. The amount of difference is 

 ascertained by reading off the reflected scale the highest and 

 lowest numbers at the turning-points of the balance coinciding 

 with a horizontal thread fixed in the telescope. 



21. As balances of precision are alone used for scientific 

 weighings, it is not proposed now to enter upon the subject of 

 the larger kind of multiplying balances, such as platform 

 weighing machines, where a unit weight is multiplied one 

 hundred fold, as these are used for commercial purposes only. 

 I may merely direct your attention to this model of a 

 centesimal balance, exhibited JSTo. 382, in which the only 

 principle involved is the most convenient modejof extend- 

 ing the length of the longer arm of the lever. It is, how- 

 ever, worth your attention as a practical illustration of the 

 mechanical power of the lever. 



22. The construction of balances of precision with knife- 

 edges on agate or hard steel plane bearings, with contrivances 

 for keeping them in position, is comparatively of recent date. 

 The best balances were previously constructed with knife- 

 edges on curved bearings enclosed in steel boxes to prevent 

 shifting. Curved bearings only are generally used for com- 

 mercial balances. 



23. You see here two balances of precision of peculiar 

 construction. The first is a balance made by Troughton early 

 in the present century, and formerly belonged to Sir James 

 South. It has recently been purchased for the Standards 

 Department. The beam is of wood, and it vibrates upon two 

 steel points resting on an agate plate. The pans are sus- 

 pended from pins placed at equal distances from the middle 

 of the line joining the two points which form the centre of 

 motion, by means of thin cords passing over the ends of the 

 beam. I propose to leave this balance to be seen in the 

 Loan Exhibition. 



24. The second is a balance recently made for the Standards 

 Department by Mr. Oertling from a design by Mr. Artingstall, 



