91
C
The following method is followed in making the observations in the magnetic hut. The small table on which the hygrometer stands is placed as near as possible in the mid- dle of the building, a small black screen being placed in front of the instrument at a distance of about two feet. The observer sits at a distance of aboft five feet from the instrument and evaporates the ether by means of a small band bellows connected to the instrument by indiarubber tubing. When dew is observed on the gold plated face of the apparatus the thermometer is read off, and again when the dew disappears, care being taken to obtain only the very lightest deposit. To effect the latter it is necessary to work the bellows very gently on nearing the dew point, otherwise the plate suddenly becomes heavily charged with dew. The rotating thermometers are laid on the floor, which is of cement, in a corner of the hut, the damp bulb thermometer having been previously wetted so that it already reads near the evaporation point. They are then rotated round the Alluard apparatus, two or three separate readings being taken. The hygrometer is then again observed and the mean of this and the previous observation is taken as a single observation." Usually three or four observations in the order named are taken and the mean of them is called “a set.” As it is necessary to clean the dew plate of the hygrometer frequently, a set of observations takes from twenty minutes to half an hour. But when the air is extremely dry the time taken may be much longer owing to the difficulty of reducing the ether to the dew point. Occasionally the temperature of the ether has been reduced previously to being used by placing it in ice or in water considerably below the air tempera- ature. But even so when the air is extremely dry and the difference between the dew point and air temperature grea', considerable difficulty still exists as the reduction of temperature caused by evaporating the ether is to some extent counter-balanced by the rise caused by the temperature of the air passed through the instrument, the ether not being evaporated fast enough and at the same time maintained at a sufficiently high level in the apparatus. The aspirator furnished with the instrument was found to be useless whenever the air was moderately dry even, it being found quite impossible to reduce the temperature of the ether quickly enough by this means. A land bellows for forcing air through the instrument was therefore substituted for it. The small tubes through which the air is passed are liable to get choke by small particles of the connecting rubber tubing finding their way into the tubes. They can only be cleaned thoroughly by taking the instrument completely to pieces which necessitates récementing tubes etc., to the ether container.
The number of sets of observations made in the various years was as follows 1887, 44; 1888, 15; 1889, 3; 1892, 18; 1893, 37; 1894, 13; 1895, 2; 1896, 30; 1897, 9; 1898, 39; 1899, 90; 1900, 48; 1901, 96; 1902, 162; 1903, 94; 1903, 112. In 1887 two sets were rejected as being palpably wrong, in 1892 eighteen sets and in 1893 twenty- two sets were rejected on account of being made mostly in the open air or in situations where light, radiation and air motion were uncontrolled,
These observations were all made by Mr. F. G. FIGG.
The following table shows the provisional corrections, in degrees Fahrenheit, to be applied to the Dew-point determined by Rotating thermometers and Blanford's tables, in order to reduce it to that indicated by "Alluard's hygrometer, for different heights of the damp bulb temperature and for various differences between the damp and dry thermometer readings.
It will be seen that the correction diminishes with increasing damp bulb temperature, with probably a change of sign at the highest readings of the damp bulb here shown. It increases with increasing differences between dry and damp thermometer readings as regards damp bulb temperatures below 60°, while above that point it seems probable that the change of correction in the same direction, is small.
The table still reveals many inequalities, and more observations are evidently required before a final discussion of the results will be possible. The opportunities for obtaining observations at the extreme points shown in the table are, however, by no means frequent.