554 ANNUAL REGISTER, 1794, 



is only at a very high temperature 

 that water exercises any sensible 

 action upon caoutchouc. I can af- 

 firm that at the hundred and 

 twentieth degree of Reaumur's 

 thermometer, [302 of Fahrenheit] 

 this affinity is not such as that the 

 water can give a liquid form to 

 caoutchouc ; and it does not appear 

 that we have any thing to fear in 

 practice from a combination be- 

 tween these two bodies, which, 

 though it really is a true solution, 

 does not take place in any sensible 

 degree but at a high temperature. 

 It is therefore, at present, easy to 

 make of caoutchouc whatever in- 

 struments it may be advantageous 

 to have of a flexible, supple, and 

 elastic, substance, which is imper- 

 meable to water at the temperature 

 of our atmosphere, and resists the 

 action of acids, as well zar that of 

 most other solvents. As to the du- 

 rability of these instruments, few 

 substances promise mpre than this, 

 because it maybe soldered afresh in 

 a damaged part. Any woven sub- 

 stance may be covered with it, it is 

 only required that the substance 

 should be of a nature not to be 

 acted upon, during the preparation, 

 either by ether or by boiling water; 

 for these tv/o agents are those which 

 appear to me to merit the prefei- 

 ence. Artists will frequently find 

 an advantage in employing ether, 

 as it requires less time j so that a 

 person may make, in a single day, 

 any tube he may have occasion for. 

 The exponr-; of etlier is very httle, 

 since it is needful only to dispose 

 the caoutchouc to adhere ; and, 

 being brought into that state, the 

 caoutchouc may be kept in a vessel 

 perfectly well closed. It v.ould also 

 diminish the eipence of the ether, 

 if, instead of washing it with a large 



quantity of water, there should b« 

 added to it only as much water a» 

 it can take up. 



On a method of escertaimng an uni- 

 versal and invaria le standard of 

 measure, by Mr. Robert Leslie, 

 watch-maker, r J London. From 

 the Repertory of Arts. 



1H ROWING together. the in- 

 eqiia'ities of solar days, a 

 mean int rval, or day, has been 

 found, and divided, by very general 

 consent, into eighty-six thousand 

 four hundred equal parts. 



A pendulum, vihraiing in small 

 and equal arcs, may be so adjusted 

 in its length, as, by its vibrations, to 

 make this division of the eartii'» 

 motion into eighty -six thousand i our, 

 hundred equal parts, called seconds 

 of mean time. 



• Such a pendulum then becomes it» 

 self a measure of determinate length, 

 to which all others may be referred, 

 as to a standard. 



But even the pendulum is not 

 without its uncertainties. 



First, the difficulty of ascertain- 

 ing in practice its centre of oscilla- 

 tion, as depending on the form of 

 the bob, and its distance from the 

 point of suspension ; and the effect 

 of the weight of the suspending 

 wire, towards displacing the centre 

 of oscillation, (that centre being 

 seated within the body of the bob, 

 and therefore inaccessible to mea- 

 surement), are sources of consider- 

 able uncerlainty. 



Secondly, both theory- and expe- 

 rience prove that, to preser\'e its 

 isochronism, it must be shorter to- 

 wards the equator, and longer to- 

 wards the poles. 



Thirdly, to continue small and 



equal vibrations through a suffi- 



' ■ . . , cient 



