472 Proceedings of the Royal Society 
of instances, to apply the principle brought forward in this paper 
to past observations. 
Observations for the ascertaining of heights must, to be satis- 
factory, include the following particulars : — 
1. Latitude and longitude of the place. 
2. The date of the observation, giving exactly the year, the 
month, the day of the month, and the hour of the day. 
3. The observation itself exactly as made ; if with a barometer 
or aneroid, the pressure to be given ; if with a thermometer, 
the boiling-point to be given, and not merely its equivalent 
in pressure. 
4. The temperature of the air in shade. 
5. The weather for two days before and after the observation, 
showing the temperature of the air, its probable humidity as 
made known by the feelings or by its effects on surrounding 
objects, the amount of cloud, the rainfall, the direction, 
veerings, shifts, and force of the wind, together with any 
striking phenomena that may occur. 
To these might be added, if possible, observations of the wet-bulb 
thermometer. 
It will be evident from these remarks that the Physical Geo- 
grapher will require the practised Meteorologist to aid him in 
settling the important physical problem of heights for large por- 
tions of the earth’s surface. 
3. Notice of a Eemarkable Mirage Observed in the Firth of 
Forth. By Dr Christison. 
Some years ago, when visiting on the 31st of May the beautiful 
scenery of Dalmeny Park, I observed about three in the afternoon, 
from the terrace on which stand the ruins of Barenbougle Castle, 
some remarkable examples of mirage on the Firth of Forth, which 
are perhaps worthy of record. 
The atmosphere was uncommonly clear in every direction, sun- 
shiny, warm, calm ; and what little wind there was came from the 
south-west. But a black, sultry-looking cloud was forming at the 
same time in the north-west, from which in an hour and a half 
afterwards, as I was on my way home to town, a severe thunder- 
