42 REPORT 1851. 



the inner one, was also formed : both were as clearly and perfectly developed as any 

 solar bow I have ever seen. They were exceedingly brilliant, and by having a dense 

 sombre back-ground, had a most magnificent appearance : several colours were per- 

 ceptible upon their outer edges, but only in a faint degree ; tinges of red and blue 

 were most distinct. The phaenomenon continued in its greatest perfection for a 

 quarter of an hour, during which time it attracted the notice of other persons, 

 as I was afterwards informed. It appeared to be but a short distance from me, and 

 was at least 20 minutes disappearing from the time of its greatest brilliancy, thereby 

 showing the slow progress made by the shower. 



Onjtlm Rise and Fall of the Barometer. By W. H. Webster. 



Description of a Sliding -Rule for Hygrometrical Calculations. 

 By John Welsh, Kew Observatory. 



This instrument has been devised with the view of facilitating the reduction of 

 observations with the dry and wet bulb hygrometer. 



The results usually deduced from the observations of the dry and wet thermome- 

 ters are, — 1st. The elasticity of the aqueous vapour present in the air. 2nd. The 

 temperature of the dew-point. 3rd. The degree of humidit}', or the ratio to com- 

 plete saturation. 4th. The weight of water contained in a cubic foot of air. 



The first of these results is deduced from the well-known formula of Dr. Apjohn, 



/"=/' • - — , where/" is the elasticity of vapour required to be found, /' the 



elasticity corresponding to the temperature of the wet thermometer, d the difference 

 between the dry and wet thermometers, and h the height of the barometer. The dew- 

 point is the temperature corresponding to the elasticity/". If /be the elasticity 



f" 

 corresponding to the temperature of the air, the degree of humidity=:'^. The weight 



of vapour is obtained from the formula 



1-375 X 258-448 xE^ 



U) =: ==== — » 



30 (I -I--002083 X <"— 32) * 



where w' is the weight in grains of the water contained in a cubic foot of air at a 

 dew-point t ; E( the elasticity of vapour for the same temperature. A factor has to be 

 applied to w' in order to correct for the increased elasticity of the vapour due to the 

 depression of the dev/-point below the temperature of the air. 



The observer, in obtaining these four results, has to make distinct calculations for 

 each observation, making six references to tables, and performing one subtraction, 

 one multiplication, and one division. By means of the instrument about to be de- 

 scribed, it is believed that the four results mentioned above can be obtained with 

 less trouble, and with as much accuracy as the observations themselves can be taken. 



The instrument is a sliding rule, about 15 inches long, and H inch broad, having 

 one sliding-piece on each side. Figure 1 in Plate I., is a plan of a portion of the 

 instrument drawn to the natural size ; the sliding-pieces being set in accordance 

 with the example given below. On the first side, the scale A is one of equal parts, 

 its argument being the elasticity of vapour in inches of mercury ; one-tenth of an 

 inch of mercury being represented by one inch in the scale. The fixed scale D shows 

 the corresponding temperature according to Dalton's table ; this scale being of course 

 an unequal one. Scales B and C, similar to D, are divided upon each edge of the 

 sliding-piece which works between A and D. In the middle of the sliding-piece are 

 three slits having bevelled edges ; on each side of these slits are divided short scales 



a, b, c, d, e, and/, representing the quantity — • — , which has to be subtracted 



from/', in order to give/". The scales a, b, c, d are adapted for each half-inch of 

 the height of the barometer from 29 to 30i ; e and / being adapted to the change 

 which takes place in the coefficient when the temperature of evaporation is below 



* See Greenwich Mag. and Met. Observations, 1842, p. xlviii. 



