468 PHILOSOPHICAL TRANSACTIONS. [annO 1754. 



that the contact of its edges with those wires cannot be observed at one view ; 

 and the least motion of the telescope, while the observer is turning his eye from 

 one wire to the other, must oblige him to repeat the observation ; whereas, by 

 this method the contact of the edges of the images is not at all affected by the 

 motion of the telescope. Whence the comparison of this micrometer with the 

 common sort, in this respect, stands thus : the one requires great steadiness in 

 the telescope, but yet it is applicable to none but such as are very difficult to 

 keep steady ; the other does not require such steadiness, though it is applicable 

 to short telescopes, which are easily managed. These advantages not only add 

 to the certainty of the observation, but assist vastly in the expedition ; for an 

 observer may make 20 observations in this way, where he could scarcely, with 

 much fatigue, be sure of one with the common micrometer. Expedition in 

 making observations must be allowed a very great advantage, in this climate, 

 where the uncertainty of the weather renders astronomical observations so pre- 

 carious, that no opportunities, even the most transient, should be let slip. An 

 instance of this was given in to the r. s., in an account of the eclipse of the sun 

 last October. As the motion of the telescope gives the observer no great incon- 

 venience in this method ; neither does the motion of the object at all disturb his 

 observation, such a motion as that of the heavens is. This gives him leave to 

 take the diameter of a planet in any direction ; or the distance between two stars 

 or planets, let their situation be how it will ; in which respect the common mi- 

 crometer is absolutely defective ; as it can give no angles, but su'^h as are per- 

 pendicular to the line of their motion ; though the diameters of the planets, in 

 other directions, is very much wanted ; it being highly probable, from the laws 

 of motion, and what we see in Jupiter, that such planets as revolve round their 

 axes, have their polar diameters shorter than their equatorial ones. 



The distances of Jupiter's satellites from each other, or from Jupiter's body, 

 cannot be measured, with any certainty, in the common way, as their position 

 is always very far from being at right angles with the line of their motion : nei- 

 ther can the moon's diameter, which must be taken from horn to horn, scarcely 

 ever be obtained that way, because it very rarely happens, that the diameter to 

 be measured lies at right angles to the line of her motion. The same may be 

 said of the distance between two stars, But this micrometer gives angles, in 

 every direction, with equal ease and certainty ; the observation being also finished 

 in an instant, without any trouble or fatigue to the observer. For as there are 

 no wires made use of this way in the field of the telescope ; so the observer has 

 no concern about any illumination. The largeness of the scale deserves also to 

 be taken notice of, as it may, in this micrometer, be increased almost at plea- 

 sure, according as the smallness of the object requires. Another inconvenience 

 attending the common micrometer is, the variation of the scale, according to the 



