34 REPORT — 1853. 



slight frost, and about noon a thin haze came over and covered the sun, which still 

 shone with considerable power ; and there were a few scattered light clouds. At the 

 time stated above, four mock suns, or parhelia, were very visible, situated at equal 

 distances in a circle round the true sun, two being in the same vertical with him, 

 and two in the same horizontal range. These displayed splendid prismatic colours. 

 Shortly afterwards two other mock suns appeared, of a pale white light, in the 

 same horizontal range with the true sun and two of the bright mock suns. Each of 

 the mock suns appeared as large as the true sun, and through him and the four hori- 

 zontal mock suns a stream of pale white light passed to a considerable distance be- 

 yond the outside ones. The au- 

 V y thor gave a diagram, as below. 



The circle in which the two outer 

 parhelia were placed was only about 

 three-fourths formed ; the diagram 

 showed it exactly twice the size of 

 the inner circle; immediately above, 

 to the north, was an inverted arch. 

 The inner circle and outer partly- 

 /^K x'/n /^^ /\~\ /V\ formed circle were of a light brown 



' ' "E colour on the outer edges, and of 



a violet-red on the inner. The in- 

 verted arch was of white light, ha- 

 ving the outer edge tinged with red. 

 The long streak was of white light, 

 which passed through the true sun 

 and the two parhelia on each side 

 of it, and to a considerable distance beyond the two outside ones. The wind being 

 scarcely perceptible, the haze hung over the sun for a long time. The phsenome- 

 non was in full splendour for upwards of two hours ; and it was a considerable time 

 afterwards before all traces had disappeared. 



On the Graduation of Standard Thermometers at the Kew Observatory. 



By John Welsh. 

 In the year 1851 the Committee of the Kew Observatory, impressed with the 

 importance, in meteorological investigations, of possessing thermometers of a better 

 class than those hitherto procurable from opticians, took steps with the view of 

 producing such instruments, under their own superintendence, for distribution to 

 institutions and individuals who might require accurate standards of reference. The 

 Committee were furnished with the information necessary to carry out their inten- 

 tions by M. Regnault of Paris, who had been accustomed to construct his own 

 thermometers by a method proposed by himself, and with an accuracy previously 

 unknown: they were also supplied, under his directions, with the requisite apparatus. 

 It has been assumed by physicists that at all temperatures, as high at least as that 

 of boiling water, the apparent expansion of mercury in a glass envelope is uniform 

 for equal increments of heat. A mercurial thermometer may therefore be called a 

 standard instrument when the divisions of its scale correspond everywhere to equal 

 volumes of the contained fluid, and when the readings are known for the tempera- 

 tures of melting ice and of water boiling under a certain barometric pressure. If the 

 tube were perfectly uniform in its bore, it would only be necessary to make a scale 

 of equal parts between the freezing and boiling-points, and to extend the division 

 above and below these points ; but as perfect tubes are in practice not procurable, 

 it becomes necessary, in dividing the scale, to make allowance for the variations in 

 the tube's capacity. These variations are obtained by carefully measuring a short 

 column of mercury (1 inch or less in length) as it is made to pass along the tube 

 by successive steps, each of which is as nearl)' as possible its own length. In 

 the thermometers constructed according to M. Regnault's process, the divisions do 

 not represent degrees of the ordinary scales of temperature, but are of an arbitrary 

 value, differing for each instrument, and requiring separate tables for each ther- 

 mometer to convert the scale readings into degrees ; the divisions at all parts of the 



