On Gravimeters. 



529 



makes one division correspond to l s '07 per diem, it is clear that this is 

 a weak point, especially as the collimator is optically indifferent. 



(6.) This " telescope " or collimator is said to be "adjusted " so as 

 to perform certain functions. Unfortunately it has no means of 

 adjustment except in a vertical plane. There is no horizontal motion; 

 nor has the object-glass a power of focal adjustment by rack and 

 pinion. It is very difficult to get a good sight of the reflected scale. 



(7.) The only dimension stated in the whole description is the 

 diameter of the "plunger' 5 (1 millim.). Neither are the weights of 

 the suspended masses stated. A precise knowledge of the joint 

 weight is necessary for the calculation, even of differential results. 

 By precise I mean to within 4 or 5 grs. 



(8.) The plunger is described as a " calibrated glass wire." I 

 imagine this to mean that the glass rod was specially made, and 

 tested at every point. I do not see any advantage in its being of 

 glass. 



(9.) " The length of glass wire immersed " — that is to say, the 

 length of liquid displaced — " is read .... to a thousandth of 

 a millimeter." The graduation of the micrometer head enables 

 hundredths of a millimeter to be read. The thousandths are by 

 estimation. But this probably far exceeds the power of observation 

 of the surface of a liquid, such as glycerine, in a glass tube. 



(10.) "The finely polished agate points" appear to be of ruby- 

 coloured glass — if ready fusion be any test. 



(11 .) The magnetic holder. I imagine this is better removed. It 

 would be nearly impossible to guarantee its action being entirely 

 horizontal. And the presence of a magnet as part of a mass, the 

 weight of which is under examination, is inadmissible. 



Should these comments give the impression that I wish to cavil at 

 the description, I must reply that it is necessary for the present pur- 

 pose. The question before us is not whether the instrument is 

 ingenious ; but whether it can be used for the intended purpose, in 

 preference to other existing instruments whose use and powers are 

 well known. 



It may be said that that question can best be answered by trying it. 

 Unfortunately this is not the case. It will already have become 

 apparent that it is one of a class of instruments in which the observa- 

 tion is nothing but the last of a series of elaborate and difficult 

 adjustments — adjustments which require patience and skill and no 

 little time, all of which would be thrown away if the ultimate obser- 

 vation should prove abortive. I will now proceed to indicate more 

 exactly what these adjustments seem to be. 



The principle on which the instrument is designed involves, as a 

 primary consideration, the angular rotation and ultimate angular 

 position of two bodies. The angles 9 and of the theory above 



