INTRODUCTION. v jj 



The mean daily results of the barometer is given on a" suitable scale 

 above and below 30 inches. 



The hours chosen for observation were those generally adopted, viz. : 

 3 A. M., 9 A. M., 3 P. M., and 9 p. M., according to civil time ; when it 

 was deemed desirable, more frequent observations were ordered. At 

 the observatories, hourly observations were made, and throughout the 

 voyage, hourly observations of the temperature of the air and water were 

 taken, both at sea and in port. A thermometer placed at the mast- 

 head was also observed at the hours above indicated ; it was sheltered 

 from accident by a small covering, which did not obstruct the free 

 passage of air : this served to check the results on deck, proceeding 

 either from reflection or the influence on the temperature arising from 

 a crowded ship. 



The barometer for the use of the observatory was an ivory float 

 gauge, made by Trougbton & Simms ; its diameter of tube was O35 in., 

 and its error on the Royal Society^ standard -007. This was considered 

 the standard : it continued in use throughout the voyage, and was in 

 perfect order at the return of the Expedition, having been used in the 

 final experiments at Washington. It was read off to thousandth parts. 



We had several standard thermometers by Troughton & Simms, 

 Dolland, and Jones, of Charing Cross, both for air and water, graduated 

 on metal and ivory to half degrees, and easily read off to tenths; the 

 length of scale. 18 inches; also several self-registering spirit ther- 

 mometers, among them, those of Six. The hygrometers in use were 

 Pouillet (a capsule), Daniels, the silver cup, and the wetted bulb. 



The Photometer and ./Etherioscope of Leslie were used. 



The rain gauge was that of Dewitt, a cone of nine inches in height 

 by six at the base ; it was measured by a rod graduated to hundredths. 

 It was placed at the end of the spanker-boom, where it was free from 

 the drip of the sails or rigging. The quantity of water that fell was 

 measured immediately after rain. 



The force of the wind was noted in the usual way by the different 

 observers. For the sake of space in the tables, the expressions have 

 been changed into numbers. 



CAPE HENEY TO MADEIRA. This passage occupied twenty-eight 

 days. The diagram (Plate I., page 18) elucidates the meteorological 

 data, by an inspection of which they will clearly appear; the tempera- 

 ture of the water due to the Gulf Stream will point out its width, and 

 the influence it exerts over the air. The movement of the barometer, as 

 well as that of the temperature, is satisfactorily exhibited in the gale of 

 the 26th and 27th of August, which was encountered on the outer edge 



