iy6 1'HILOSOPHICAL TRANSACTIONS. [aNNO 1/82. 



.side, the cold of the preceding night ; and by that on the right, the heat of the 

 day. These I minute down, and then apply a small magnet to that part of the 

 tube against which the indexes rest, and move each of them down to the surface 

 of the mercury : thus, without heating, cooling, separating, or at all disturbing 

 the mercury, or moving the instrument, may this thermometer, without a touch, 

 be immediately rectified for another observation. When I wish to put the ther- 

 mometer out of my hand, without hanging it up, I have a stand to place it on ; 

 for if the mercury presses against the index, while the instrument lies in an hori- 

 zontal position, it is in danger of passing by it, which is avoided by keeping the 

 thermometer in a position nearly vertical. 



XI. On the Parallax of the Fixed Stars. By Mr. Herschel. F. R. S. p. 82. 



To find the distance of the fixed stars has been a problem which many eminent 

 astronomers have attempted to solve ; but about which, after all, we remain in a 

 great measure still in the dark. Various methods have been pursued without 

 success, and the result of the finest observations has hardly given us more than 

 a distant approximation, from which we may conclude, that the nearest of the 

 fixed stars cannot be less than 40 thousand diameters of the whole annual orbit 

 of the earth distant from us. Trigonometry, by whose powerful assistance the 

 mathematician has boldly ascended into the planetary regions, and measured 

 the diameters and orbits of the heavenly bodies, for want of a proper base, can 

 here be but of little service ; for the whole diameter of the annual orbit of the 

 earth is a mere point when compared to the immense distance of the stars. 

 Now, as it is not in our power to enlarge this base, we can only endeavour to 

 improve the instruments by which we measure its parallax. 



There are two things requisite for measuring extremely small angles with 

 accuracy. First, that the instrument we use for this purpose, be it quadrant, 

 sector, or micrometer, should be divided and executed with sufficient exactness ; 

 and, secondly, that the telescope, by which the observations are to be made, 

 should have an adequate power and distinctness. On the first head, the great 

 improvements by instrument-makers have hardly left us any thing to desire : we 

 can now measure seconds with almost as much facility and truth as former ob- 

 servers could measure minutes ; nor does it seem impossible to go still further, 

 and divide instruments that would show thirds with sufficient accuracy. It is in 

 the latter, or optical part, we find the greatest difficulty. To see a single second 

 of a degree with precision, requires a telescope of very great perfection ; there- 

 fore, supposing the mechanical part of an apparatus well executed, it will still be 

 necessary to try how far the power of our telescope will enable us to ascertain 

 with confidence the division or number of seconds it points out. If on trial we 

 find that our instrument will give us the same measure within the second, every 



