for determining the Length of the simple Pendulum. 101 



not at all bearing on the point in question. I ought not how- 

 ever to omit mentioning that, in order to render the respective 

 knife edges synchronous, I caused a hole to be drilled at each 

 end, and inserted some pieces of lead. But this, as far as our 

 present knowledge of the pendulum extends, will have no other 

 effect than to increase the specific gravity of the pendulum; 

 and consequently to diminish, in a slight degree, the correction 

 for the reduction to a vacuum; and which moreover will equally 

 affect all the knife edges. The two ends are symmetrical. 



As these are the first and only experiments (as far as my 

 information extends) that have been made on the convertible 

 pendulum, since those by Captain Kater, they may probably 

 be the means of inducing others to take up and investigate the 

 subject. I consider them, at present, only 2i?, preliminary : 

 since it appears to me to be a waste of time to attempt any 

 much greater degree of exactness, or (o aim at any definitive 

 results, when discordancies of such magnitude present them- 

 selves at the commencement of our inquiries, and for the ex- 

 istence of which we are unable to account. I am aware 

 that M. Bessel has lately made a number of very inter- 

 esting and valuable experiments on pendulums of various kinds, 

 and under a variety of different circumstances : and that, from 

 the result of those experiments, there is reason to believe that 

 the common correction for the reduction to a vacuum is not 

 the same for the two positions of the convertible pendulum. 

 But I am not awai'e that he has been able to deduce any for- 

 mula whereby we may determine the true correction which 

 ought in every case to be applied. The public, however, wait 

 with much impatience for the publication of his important dis- 

 coveries. Probably the only accurate mode of determining 

 the amount of this correction, is by actual experiment : — by 

 swinging every convertible pendulum in both positions of the 

 knife edges ; first in the open air, and afterwards in vacuo: and 

 I believe that a convertible pendulum is about to be treated in 

 this manner, by means of a new apparatus now erecting at the 

 Royal Observatory at Greenwich. 



Whether the result of those experiments may tend to re- 

 move the discordancies here alluded to, I cannot pretend to 

 say. If they should fail, it will be evident that the pendulum 

 will not afford us the means of obtaining a correct and per- 

 manent standard, and which, if ever lost or destroyed, may be 

 reproduced upon certain and unerring princijiles : but that we 

 must seek lor some other mode of deducing so invaluable a 

 measure. 



In the fourth volume of the Base diiSystcme Mcirique, page 

 587, M. Biot has given the mean result of the experiments of 



MM. Horda. 



