NATURAL PHILOSOPHY. 175 



adverting to the use of the spirit-level for adjustment of astronomical in- 

 struments on land, Prof. Smyth observes : " On shore nothing: can be more 

 convenient than this use of the level; but its adaptation to sea service .is not 

 easy. For, first, on account of the constant movement of the vessel, never 

 for the one-thousandth of a second stationary, we cannot first adjust the 

 level and then look at the star, but must see both the star and the level sim- 

 ultaneously in the field of the telescope. Let us accomplish this before going 

 into further and residual difficulties. The sextant or angular measurer at sea 

 is made to allow of two objects being seen at once, the star, and the referring 

 point at a great or infinite distance, usually the horizon of the sea; but in the 

 present case the level-bubble is to take place of the horizon. We must, there- 

 fore, 1st, put it at an infinite distance by means of a collimator; 2nd, have 

 it to be looked at in a horizontal direction ; and 3d, have the bubble something 

 very notable. Here a level is illuminated by artificial light thrown through 

 it from below; above is a diagonal mirror, and in front a lens, in whose so- 

 lar focus the bubble is. An eye therefore in front sees the bubble in the hor- 

 izontal direction, at an infinite distance, and very visibly, because where the 

 fluid curls up to form the edge of the bubble, it scatters the upward light so 

 completely that none reaches the eye, and the bubble looks as if it had a 

 painted black margin. If we place the eye partly in front of the collimator, 

 and partly off, we may of course see both the level-bubble and something 

 else ; and in sextant observations ma}', by moving the proper mirror, sec a 

 star and the level-bubble in the field together. To make a practical use of 

 the level so arranged, we may either have it attached to the frame of the sex- 

 tant, or have it on a stand in front of the sextant. But in either case the 

 bubble must be brought to its own fiducial mark at the instant of an}' obser- 

 vation. Herein is the first of our practical difficulties ; for if the level be on 

 a stand, and that on the ship's deck, the bubble is rolling from end to end of 

 the tube with every roll of the ship; and if the apparatus be fastened to the 

 sextant and held by hand, no man has power or tact to hold it level. Some 

 natural principle must be brought in then to assist man's hand. And such a 

 principle is found in the level-bubble itself, which will always be at the sum- 

 mit of the arc. If, therefore, we can refer our observations, not to the fidu- 

 cial mark, but to the bubble itself as it moves apparently with the rolling of 

 the ship, we shall be able to measure altitudes accurately at sea; and herein 

 are the chief of my novelties. If we make the radius of curvature of the 

 level, and the solar focus of the collimator equal, then if the whole is tilted 

 one degree the bubble viewed in the diagonal mirror runs in the opposite direc- 

 tion to the same angular extent, by reason of the equality of the level radius, 

 and the focal length of the collimator, or radius of the sphere on which the 

 optical picture is formed. The result of these compensations is, that tilt the 

 case of the level up or down by any quantity within the angular extent of 

 the level tube, you will always see the bubble in the same true horizontal 



direction When carrying out the principle in practice, if the level is 



of 12 feet radius, a collimator 12 feet long would be an inconvenient append- 

 age to a sextant; but the focus may then be lengthened virtually by insert- 

 ing one or more concave lenses with a convex of much shorter focus. Xow 

 put everything together; and for an observation, in place of the ordinary 

 sextant, use my reflecting circle, the level being attached to the same stand. 

 Looking into the telescope, you see the star in the upper mirror, which moves 

 with the circle, and gives the angle moved through on the vernier. The 

 lower mirror, fixed on the frame of the instrument, shows the bubble of the 



