170 ANNUAL OF SCIENTIFIC DISCOVERY. 



level, along with the star, in the telescope. The observer sees the bubble in 

 the middle of the field, and the star in its centre. If the stand be tilted by a 

 small angle, the star ascends or descends in the field, but so also by the same 



angle does the peculiar level-bubble In other words, though the field 



of view, and all fiducial marks of the telescope and the level-tube vary and 

 err, yet the level-bubble always continues to show the true horizontal direc- 

 tion ; it preserves the same angular distance from the star, and all that we 

 have to do in making an observation (no matter whether the base be quite 

 level or not and herein is the difference from the ordinary use of a level on 

 land), is so to move the upper mirror of the circle as to make the star come 

 on to the bubble, and then read ofT the verniers. Once on the bubble, the 

 star will remain there as long as the support, or ship's deck, does not roll 

 through a greater angle than the level-tube comprises, or docs not roll in a 

 quick, jerking manner. For these larger deviations the principle of the sta- 

 bility of the free revolver must be applied, as elsewhere described. (Sec 

 'Ast. Soc, Notices,' xvii. 40.) On looking into the collimator, all the trans- 

 mitted light is blocked out, except the central line. This is to keep the field 

 dark and appropriate for star observations. The self-adjusting motions of 

 the bubble then take place along the luminous lane, and laterally an approx- 

 imate correction is shown by the approach of the bubble to one or the other 

 side of the said lane." The inventor also adds, with reference to the prac- 

 tical construction of the level: "Some practical men would exclaim, on 

 hearing of a level of one foot radius, that it must be so sluggish as to be good 

 for nothing; yet it is less sluggish; and the nature of sluggishness in levels 

 has been much misunderstood, vi/.., mixed up with amount of motion of the 

 bubble as depending on lenirth of ratlins of the tube. Make treacle thelluid 

 of levels, and they will all be sluggish; long or short radii, ?'. r., after a cer- 

 tain angular tilt, the bubbles will be moving slowly for a long time before 

 coming to their new point of rest. Put ether into a tube of long radius : it 

 moves quickly in the above case from want of sluggish nature, and moves 

 far, by reason of the long radius. Tut ether into a tube of very short radius : 

 men say, How sluggish, because it does not move far, measured in the linear 

 way. But they should rather ascertain which bubble comes to its new place 

 of rest soonest; and if by reason of short radius they cannot easily see 

 Avhat arc one bubble moves through, they can increase the radius optically by 

 magnifying. Now, by mere optical magnifying, the bubble has no mechan- 

 ical friction added to its motion ; but if the magnifying be by increasing the 

 radius of the level, the bubble has a greater mechanical task of walking along 

 so much more in length of glass surface. Hence it may be shown that these 

 new short-radius levels, viewed through magnifiers, are as accurate, and 

 quicker in their motions of angle, than the usual ones of long radii looked at 

 with the naked eye. All fluids have more or less of the sluggishness or 

 stickiness so greatly developed in treacle: alcohol has less than water; ether 

 the Germans found to have less than alcohol; and chloroform I have found 

 to have less than ether, while it has the great advantage (seeing that the end 

 of the tube must be hermetically sealed with a blow-pipe flame) over both 

 ether and alcohol, of not being inflammable." He also mentions that prac- 

 tically a good range of arc is 5, and a good radius, 9 feet. 



At the close of this communication Prof. Powell presented a detailed 

 account of the principle of the construction of Prof. Smyth's new instrument. 

 To render such a description complete, Prof. P. stated, that it appeared to 

 him, that a more precise elucidation of the optical principle involved was 



