SCIENTIFIC NOTES. 143 



voyage from Liverpool to India and back, the time, after correc. 

 tions are applied, will be very small. It is only necessary to note 

 the temperature regularly during the voyage, and from this and 

 the formula supplied by Mr. Hartnup, the daily rate is at once 

 known. Mr. Hartnup is very anxious to have the discovery 

 tested as widely as possible, and I shall be very glad to show any 

 one interested the result of experiments so far made. 



The late Sir Charles Wheatstone has introduced a very curious 

 instrument which he calls a polar clock. One has gone with the 

 Polar Expedition, by which, if the sky is clear, the time of the day 

 may be told within two or three minutes by the polarization of the 

 sky. The instrument is so arranged that the sun's position, and 

 therefore the time of day, is indicated as soon as it is turned 

 sufficiently to remove polarization. 



I did not learn much that was new about spectrum analysis. Dr. 

 Vogel, who is appointed to the new Physical Observatory at Berlin 

 (Potsdam), is now engaged making a map of solar lines which will 

 contain three times as many lines as the existing maps. Dr. Vogel 

 is working at the Berlin Observatory, for only the foundations of 

 the Potsdam building are laid, and most of the instruments have 

 yet to be made. Professor Piazzi Smythe thinks that he has 

 discovered some new lines in the spectrum of the atmosphere just 

 before rain. Dr. Huggins, with the beautiful apparatus furnished 

 to him by the Poyal Society, was unable to do anything all the 

 time I was in England, simply because the perfection of the appa- 

 ratus demands such accuracy in the driving clock of the telescope 

 as has never before been obtained ; and the clock was away in the 

 maker's hands to see if he could make it do what was wanted, viz., 

 keep the 15-feet telescope directed so steadily to a star that the 

 image in the focus should not vary from its position by the thick- 

 ness of an ordinary cobweb, or, in other words, that it should keep 

 the same time as the earth on its axis, and not vary a tenth part 

 of a second. Indeed, the great effort now is to improve the 

 spectroscope and the means of using it; the early forms, which 

 increased the power by increasing the number of prisms, always 

 gave a hazy and unsatisfactory definition under high power, owing 

 to the difficulty of grinding the faces of the prisms perfectly flat. 

 And now the effort to improve is taking two directions — first to 

 increase the density of the glass, which M. Eeil has done until it 

 is equal to bisulphide of carbon. But here a new difficulty pre- 

 sented itself — such glass tarnishes and turns black in a few days. 

 Opticians, and especially Schroeder, of Hamburg, to get over this 

 difficulty, face the prisms with crown glass, and the large spectro- 

 scope made for Potsdam, and which Dr. Vogel is now using for a 

 new map of the solar lines, is of this form ; it shows nine lines 

 between the D lines, and is a very fine instrument. 



