58 Questions for Solution rdathig to Meteorology^ 



red of the interior arch, numerous green and purple series, form- 

 ing narrow contiguous arches, well defined, and perfectly con- 

 centric to the principal arch. Of these supplementary arcs (for 

 that is the name given to them), the theory of Descartes and 

 Newton makes no mention, and indeed it cannot even apply to 

 them. 



The supplementary arcs appear to be an effect of luminous 

 interferences. These interferences cannot be produced but by 

 drops of water of a certain smallness. In order that the phe- 

 nomenon may appear with some degree of brightness, it is ne- 

 cessary that the drops should not only be of the proper size, 

 but that at least the greater part of them should be equal in 

 their dimensions, almost with mathematical accuracy. If, there- 

 fore, the rainbows of equinoctial regions are never attended with 

 supplementary arcs, the circumstance would afford a proof that 

 the drops of water which there issue from the clouds are of 

 larger size, and more unequal dimensions, than in our climates. 

 In our ignorance of the causes of rain, this fact would by no 

 means be void of interest. 



When the sun is low, the upper portion of the rainbow is, on 

 the contrary, very much elevated. It is towards this culminat- 

 mg region that the supplementary arcs shew themselves in great- 

 est splendour. Leaving that point, their colours rapidly become 

 feeble. In the lower regions, near the horizon, and even con- 

 siderably above it, no traces of them are ever seen, at least in 

 Europe. 



It must needs be, then, that the drops of water, during their ver- 

 tical descent, have lost the property which they at first possessed ; 

 they must have laid aside the conditions necessary for eflScacious 

 interferences, which they do by having become much enlarged. 



Is it not curious, it may be remarked in passing, to find in an 

 optical phenomenon, in a peculiarity merely of the rainbow, — the 

 proof that in Europe the quantity of rain must be so much the 

 less the higher we place the vessel in which it is to be received!* 



The increase in the size of the drops is, no doubt, to be as- 



• In the observatory at Paris, there are two receptacles in which rain water 

 is collected ; one of them on the terrace, the other in the court, eighty-six feet 

 lower than the first. In the course of a year the reservoir in the court re- 

 ceived eight-hundredths more water than that placed on the terrace. 



