to the Antarctic Re<iions. 215 



*■£> 



one experiment only can be made, the most advantageous hour should 

 be selected for the purpose. Now it is probable that the minimum 

 temperature of the air in the ^^ hours may correspond with the 

 minimum temperature of the dew-point ; and for the attainment of 

 a mean result, the time of the highest dew-point should be selected, 

 which would not differ much from 3 p.m., at which hour the obser- 

 vation should on no account be omitted. The hygrometer should 

 also be observed, if possible, at 9 a.m. and 9 p.m., but the minimum 

 temperature might probably be substituted for the 3 a.m. observa- 

 tion without any material error. 



Occasional observations of the dew-point under peculiar circum- 

 stances, as for instance in the inhabited apartments of houses or be- 

 tween the decks of the ships when laid up in their winter quarters in 

 the polar regions, could not but afford information of high practical 

 importance. 



All the ether of the dew-point hygrometer should be driven by 

 the warmth of the hand from the covered ball into the uncovered, 

 previously to an observation, and the ether should be dropped from 

 a dropping-bottle very slowly upon the former. The temperature of 

 the interior thermometer should be carefully noted upon the first ap- 

 pearance of the ring of dew upon the black bulb, and also its tem- 

 perature upon its disappearance : the mean of the two observations, 

 should they differ, may be entered as the dew-point, together with 

 the temperature of the air by the exterior thermometer. 



The wet-bulb hygrometer can be observed without difficulty, by 

 mere inspection, and the observation should never be neglected at 

 the regular hours. It is probable that the temperature of evapora- 

 tion thus ascertained may afford the means of accurately determining 

 the dew-point, and of solving all the points of hygrometry ; but until 

 all the necessary corrections shall have been agreed upon, one of the 

 most essential requisites must be its frequent and accurate compa- 

 rison with the dew-point, directly ascertained. 



The hygrometers should be placed in the observatory, near to the 

 standard thermometer, with which they should be frequently com- 

 pared. 



6. Vanes, Anemometers, and Rain Gauges. 



The magnetic observatories and the Antarctic Expedition have 

 been furnished with Osier's self-registering anemometer and rain- 

 gauge. 



In this instrument the direction of the wind is obtained by means 

 of the vane attached to the rod, or rather tube, that carries it, and 

 consequently causes the latter to move with itself. At the lower ex- 

 tremity of this tube is a small pinion working in a rack, which slides 

 backwards and forwards, as the wind moves the vane ; and to this 

 rack a pencil is attached, which marks the direction of the wind on 

 a paper ruled with the cardinal points, and so adjusted as to progress 

 at the rate of 1 inch per hour, by means of a clock ; the force is at 

 the same time ascertained by a plate 1 foot square, placed at right 



