Hydrography^ and the Art of Navigation. 25 



udometer. It seems to us, therefore, expedient to recommend 

 its commander to cause it to be placed on the stern of the vesseU 

 in such a situation that it can neither receive the rain collected 

 by the sails, nor that which falls from the cordage. 



Navigators will add greatly to the interest of these observa- 

 tions, if they determine at the same time the temperature of the 

 rain, and the height from which it falls. 



In order to obtain the temperature of the rain with some de- 

 gree of accuracy, it is necessary that the mass of the water should 

 be considerable, relatively to the size of the vessel which con- 

 tains it. A metal udometer will not answer for this purpose. It 

 would be infinitely preferable to take a large funnel of some 

 light stuff, very dense in the texture, and to receive the water 

 which runs from the under side of it in a glass of small dimen> 

 sions, containing a small thermometer. So much for the tem- 

 perature. The elevation of the clouds in which the rain is formed 

 cannot be determined but during the time of the storm ; then, 

 the number of seconds which elapse between the appearance of 

 the flash and the arrival of the sound, multiplied by 337 metres 

 — the degree of rapidity with which sound is propagated — gives 

 the length of the hypothenuse of a right-angled triangle, whose 

 vertical side is precisely the height required. This height may 

 be calculated, if by means of a reflecting instrument we estimate 

 the angle formed with the horizon by the line which, parting 

 from the eye of the observer, terminates in that quarter of the 

 cloud where the lightning first shewed itself. 



Let us suppose, for an instant, that rain falls on the vessel 

 whose temperature is below that which the clouds must possess, 

 judging from their height and the known rapidity of the decrease 

 of atmospheric heat ; every one will understand the place which 

 such a fact would occupy in meteorology. 



Let us suppose, on the other hand, that during a day of hail 

 (for hail sometimes falls in the open sea), the same system of ol>- 

 servations had proved that the hailstones were formed in a region 

 where the temperature of the atmosphere was higher than the 

 point at which water congeals, and science would thus be en- 

 riched with a valuable result to which every future theory of 

 hail must necessarily be accommodated. 



We could adduce many other considerations to demonstrate 



