1823.]; Prof. Henslow on the Deluge. 347 



and were increased greatly upon the earth." All which seems 

 to imply an extraneous supply of water ; for although the atmo- 

 sphere at all times contains a certain quantity of aqueous vapour, 

 yet this would not be sufficient to answer the demand. We 

 know that the weight of the whole atmosphere is equivalent 

 only to a depth of water of about 34 feet, and this is made up 

 of atmospheric air, water, and different gaseous mixtures. We 

 must, therefore, look for some other cause, which has ceased to 

 operate since the supply was furnished ; and here of course 

 nothing but conjecture can be offered. With many of my pre- 

 decessors in this department, I must have recourse to one of 

 those bodies which have so often been considered as the proba- 

 ble cause of the Deluge, though the mode in which they have 

 been supposed to operate in effecting that event has as often been 

 refuted or ridiculed. But before we attempt to enlist so myste- 

 rious an agent into our service, let us inquire what it is we 

 actually know respecting the nature of comets. 



Of this we can judge only from the astronomical and optical 

 phenomena which they present. It is certain that they are 

 material substances, and it appears universally conjectured by 

 the most accurate observers, that they are in great part, if not 

 wholly, composed of aqueous vapour. Some comets present a 

 nucleus encircled by this vapour ; others have no nucleus at all. 

 As they approach the sun, they become brighter ; the luminous 

 train or tail, when it exists, becomes enlarged and more briUiant, 

 and when the comets have arrived at their perihelion, their lustre 

 is sometimes found to exceed that of the planets. In their retreat 

 from the sun, these phenomena are reversed, till at length the 

 light reflected from them is too trifling to be any longer visible. 

 Although the opinions which have been promulgated concern- 

 ing the tails of comets differ materially with respect to what may 

 be the nature of their substance, yet they are all compatible 

 with the idea of their nuclei being composed of aqueous parti- 

 cles. One opinion is that these tails are the light of the sun 

 refracted through the comet acting like a transparent lens ; but 

 this idea seems to have been satisfactorily refuted by subsequent 

 observation. Others suppose them to be the vapour of the comet, 

 either driven behind it by the impulse of the sun's rays, or raised 

 by the heat of the sun ; the latter opinion, which was held by Sir 

 Isaac Newton, seems also compatible with the idea of the comet 

 transmitting the rays of light, since the heat would be greatest, 

 and consequently the vapours lightest, along the train of light 

 reaching from the nucleus to the focus of this astronomical lens. 

 However this may be, let it be granted as highly probable, that 

 some comets are composed of aqueous particles, which at a 

 distance from the sun will probably concrete into the form of a 

 globe of ice, and on approaching him will either be wholly or in 

 part converted into vapour. What will be the effect of such a 

 comet approaching within the sphere of the earth's attraction ? 



