152^ Natural-PkUosopkical Collections. 



being read off by the two verniers, we have the altitude deduced from the mean 

 of four readings. 



To take horizontal or oblique angles, the vertical column is unscrewed from 

 the stand, and the ball and socket joint screwed in its place. 



With this instrument the time may be deduced from the sun's altitude, taken 

 under favourable circumstances, to within three-tenths of a second of the truth ; 

 and the latitude deduced from a single observation of the pole-star will seldom 

 differ more than twenty seconds from the truth, or, by a mean of seven or eight ob- 

 servations, it may be determined to five seconds. 



The three horizontal threads with which the telescope is furnished, make it a 

 very efficient equal altitude instrument. 



The great advantages of this instrument over a sextant, to persons travelling 

 on land, are the facility and expedition with which it can be used, requiring none 

 of those troublesome adjustments which in all reflecting instruments are necessary ; 

 being a perfect circle, with two verniers, any errors of eccentricity are corrected ; 

 it can be used when the sun or star is in the zenith, which gives it an immense 

 advantage in tropical latitudes over the sextant, and it renders unnecessary that 

 troublesome auxiliary, an artificial horiaon. If the sun's altitude be taken, and 

 a different limb be brought to the horizontal wire, when observing with the in- 

 itrument turned half round, the mean will give the apparent altitude of the sun's 

 centre, consequently there will be no allowance necessary for semi-diameter. 



All the parts essential to accuracy are finished in the best manner ; and those 

 parts, where a high finish would only add to the expense, are left in a rough 

 state, and painted. The whole is packed in a mahogany box, seven inches long, 

 by four wide, and three deep, which also contains a zenith eye-piece. Mr. Ro- 

 binson, of Devonshire Street, Portland Place, is the maker, and he charges seven 



This instrument, when intended for more accurate surveys, has a horizontal as 

 well as a vertical circle. The horizontal circle is furnished with three equidistant 

 verniers, and a lower telescope, which, when directed to a fixed object, indicates 

 any accidental derangement which might take place in the position of the instru- 

 ment. The price of the instrument thus constructed is ten guineas — Brands' t 

 Jowmal. 



On the Motion of Currents in Liquids..— M. Dutrochet has made some sin- 

 gular remarks on the influencing causes of the motion of currents in liquids, and 

 has found light to affect them considerably. He finds that difference of tempera- 

 ture is the efficient cause, and that 1 -800th of a degree of difference is sufficient, 

 when aided by light. In the absence of light such motion will cease. When 

 the windows of the experimental room are closed so much as only to admit light 

 enough to distinguish, whether the circulating motion continues or not, it soon 

 ceases ; when re-opened, the motion recommences. When completely suspend- 

 ed by the absence of light, if the table be struck with slight blows, the vibration 

 communicated immediately causes the motion to recommence ; when the liquid 

 was again at rest, the sound of a violoncello, or bell, was occasioned, and the vi- 

 brations communicated to the liquid, again caused it to acquire circulating mo- 

 tion. Hence it would appear that the vibration of the particles of a liquid favour 

 the circulating motion which a slight difference of temperature is competent to 

 produce ; that this previous vibration is a necessary circumstance, and that light 

 consequently only acts by producing it amongst the particles of the fluid. From 

 whence M. Dutrochet concludes, that, in the phenomena of circulating motion in 

 liquids, two causes operate — the efficient one, difference of temperature, the ac- 

 eessory one, light, or any other circumstance which can commimicate feeble vi- 

 brations to the particles of fluid.— iieuwe Encyc. Jan. 1830. 



