112 REPORT — 1894. 



during the rest of the month, so that on several nights the usual daily weather 

 report could not be wired to the newspapers. The thunderstorm of July 7-8, 

 though very severe, was foi-tunately unaccompanied by any damage. 



At Fort William the mean atmospheric pressure at 32° and sea level 

 was 29"871 inches, and at the top 25"324 inches, the difference being thus 

 4'547 inches. The lowest pressure at the top for the year was 23-888 

 inches in December and the highest 26 "003 inches in April, the difference 

 being 2"115 inches, being considerably above the average difference. This 

 large difference was due to the low reading in December, which was an 

 altogether exceptional month as regards the almost continuous saturated 

 state of the atmosphere, and to the high readings which accompanied the 

 anticyclonic weather of the spring and early summer. In truth, the 

 monthly means were uninterruptedly above the average for the six months 

 from March to August, the mean excess for the half-year being so much 

 as O'lOl inch above the average, an excess only exceeded in 1887, the 

 Jubilee year, when the mean monthly pressure was uninterruptedly above 

 the average from February to July, the mean excess being 0'14:4 inch. 

 This period was also strongly anticyclonic. 



The important hygrometric research carried on at the High and Low 

 Level Observatories and described in the Committee's last two Reports 

 to the British Association has been continued. During the past year 

 Mr. Herbertson has conducted the observations with the assistance of 

 Mr. Angus Rankin, First Assistant at the Observatories, and of Mr. F. J. 

 Hambly, F.C.S., F.I.C., Assistant Lecturer on Chemistry at University 

 College, Dundee ; and of Mr. Marr, Demonstrator of Botany in the same 

 College. 



An Assmann aspiration psychrometer was read for dry and wet bulb 

 temperatures, in addition to the thermometers in the Stevenson screen. 

 The dust particles in the air Avere counted, and the general weather con- 

 ditions of each experiment were noted. Nearly 100 experiments were 

 made at both observatories, of which 57 were synchronous. 



A comparison of the readings of the ventilated thermometers with 

 those of the screen only shows that in calm, or virtually calm, weather 

 the wet-bulb in the Stevenson box is much nearer the dry-bulb reading 

 than in the Assmann aspiration psychrometer. When no measures are 

 taken for causing an air current to pass the thermometer bulbs, all 

 readings made in calm or light airs require to be neglected in hygro- 

 metric work. Under ordinary conditions, the total amount of water 

 vapour in the air does not vary much in a fine day. 



A discussion of the simultaneous observations at high- and low-levels 

 brings out some very interesting results. On September 11, 1893, with a, 

 normal temperature gradient between the two observatories, the water- 

 vapour remained fairly constant at both places all day, there being an 

 excess of about 1-5 gramme per cubic metre at the lower station. On 

 September 4 the summit temperature was only from 2° to 7° lower than 

 at Fort William, instead of 16°'0 the normal difference ; and on this 

 occasion the difference between the quantities of water- vapour was as great 

 as from 6-67 to 4"60 grammes per cubic metre. This great variation was 

 almost entirely due to changes in the amount of water-vapour in the 

 upper air, since there was a steady increase of vapour from 9*15 grammes 

 per cubic metre at 9 a.m. to 10'56 at 2 p.m., and 11 '40 at 7 p.m. at the low- 

 level station ; whereas the vapour at the summit was 2*72 at 9 a.m., 5-96 

 at 2 p.m., 392 at 5 p.m., 5-55 at 7 p.m., and 5-89 at 9 p.m. ; the maximum 

 at the summit at 2 p.m. being evidently caused by an uprush of moister 



