ON DIVIDING ASTRONOMICAL INSTRUMENTS. 47 



this 9alt, of the finest quality, yielded by distillation, exclu- 

 sive of the acid product and coal, 



Pure dry carbonate of potash, • • • • 350 parts 



Tartriteoflime 6 



Silex 1*2 



Alumine 0*25 



Iron mi lgled with manganese • • • • 0*75 



X, 



On an Improvement in the Manner of dividing Astronomical 

 Instruments. By Henry Cavendish, Esq. F. R. S.* 



HE great inconvenience and difficulty in the common inconvenience 

 method of dividing arise from the danger of bruising the ofthecoramon 



,....', . , „ , • , ° r method of di- 



divisions by putting the point ot the compass into them, and viding instru- 

 from the difficulty of placing that point midway, between me nts. 

 two scratches very near together, without its slipping to- 

 wards one of them ; and it is this imperfection in the com- 

 mon process, which appears to have deterred Mr. Trough- 

 ton from using it, and thereby gave rise to the ingenious 

 method of dividing described in the preceding part of this 

 volume f. This induced me to consider, whether the above- Method of re- 

 mentioned inconvenience might not be removed, by using a mod yi n g ***»• 

 beam compass with only one point, and a microscope instead 

 of the other; and I find, that in the following manner of 

 proceeding, we have no need of ever setting the point of the 

 compass into a division, and consequently, that the great 

 objection to the old method of dividing is entirely removed. 



In this method it is accessary to have a convenient support Apparatus for 

 for the beam compass : and the following seems to me to be dividing astro, 

 as convenient as any. Let C C C (Plate I, Fig. 1,) be the me^tsde^Tbl* 

 circle to be divided, B B B a frame resting steadily on its ed. 

 face, and made to slide round on it with an adjusting mo- 

 tion to bring it to any required point: d$ is the beam com- 



* Phitos. Trans, for 1809, p; 221. 



t See Journal, vol. XXV, p. 1, and 10*. . 



pass, 



