281
Table XVIII exhibits also the height to which one must ascend in order to have the monthly mean temperature lowered one degree. The figures have been obtained from the data in Table II. Of course the fall of temperature within some ten or twenty feet of the ground is much greater especially on a hot day, but the effect of this is eliminated by observing the temperature at the same height above ground at both the upper and the lower station. The results obtained for each month during the past three years are exhibited below. The results for 1884 have been re-computed by aid of the true air-temperatures given above. It is seen that the height varies from about 200 to about 700 feet, but of course the results in individual instances vary to a somewhat greater extent, and this must often make the true astronomical refraction different from that obtained from the tables, Moreover the temperatures adopted in the construction of the tables may have been very different from the true air-temperatures and when subsequently these tables are used in other places to clear the observations from the effects of refraction, it is very doubtful whether the thermometers are exposed in exactly the same way as where the tables were constructed and even so the process is not strictly accurate if the true air-temperature is not exactly determined. It would therefore be of importance to have the rotating thermometer introduced in astronomical observatories. This instrument is particularly well adapted for occasionally determining the temperature during the night. No account is taken of the effect of the barometric gradient in the neighbourhood, which tends to make the value of the re- fraction different in different azimuths. The rates at which the temperature falls with increasing height in a cyclone and in an anticyclone are different. For these reasons the refraction is one of the most uncertain elements in practical astronomy.
1884.
1885.
1886.
Mean.
January,
.280
224
299
268
February,
289
251
294
278
March,
.533
397
656
529
April,
.437
416
406
420
May,
..341
275
294
303
June,
271
275
258
268
July,..........
...........251
294
267
271
August,
..255
328
280
288
September,
......262
310
280
284
October,
.....258
294
280
277
November, ....
..271
284
234
263
December,
..258
322
222
267
Mean,
...309
306
314
310
The speed with which the temperature falls on ascending in the atmosphere is seen to be a function of the humidity. The change is much smaller in damp than in dry weather.