'14 
HINTS TO TRAVELLERS. 
which the mercury that may be driven beyond the scale by over-heating 
will not fill; otherwise any accidental over-heating will break the bulb. 
The unavoidable shaking or any sudden shock during travelling is 
apt to cause the mercury column to separate, and a portion of it may 
be driven to the top of the tube, where it may remain unless looked for 
and brought back. Hence it is important to see that the top of the 
bore of the tube is visible, and not covered by any attachment holding 
the tube to a wooden frame. Thermometer readings are absolutely 
valueless unless the whole of the mercury fills the bulb and forms a 
continuous column in the stem. To bring a broken column together 
the best plan is to invert the thermometer, if necessary shaking it 
gently, until the mercury flows from the bulb and entirely fills the 
tube, leaving a little vacant dimple in the mass of mercury in the 
bulb. When this is done, the thermometer should be brought into 
its normal position bulb downwards, and the column will usually be 
found to have united. If this method does not succeed the thermometer 
may be held in the hand by the upper end, raised to the full stretch 
of the arm, and swung downwards through a wide arc with a steady 
sweep. I have never known this method to fail. 
Thermometer Screens .—It is usual at fixed stations to expose the ther¬ 
mometer to the air by hanging it in a screen made of louvre-boards 
so arranged that the air penetrates it freely while the direct rays 
of the sun are cut off. The Stevenson screen, constructed on this 
plan with a door opening on the side away from the sun, is well 
adapted for use in temperate countries; but it is too cumbrous to carry 
on a journey and does not afford sufficient ventilation for use in tropical 
countries. An excellent substitute is the canvas screen devised by the 
late Mr. H. F. Blanford, which consists of a bamboo frame carrying the 
thermometers (with their bulbs four feet from the ground). The whole 
structure is five feet high, and is sufficient for any places where the wind 
is moderate. It is constructed of bamboos or rods of light wood, cords, 
and canvas, which may readily be made up before starting, and it is 
easily renewed or repaired. The canvas roof should be triple or quad¬ 
ruple according to the thickness of the material. Such a screen will 
afford sufficient protection at night, or even in the day, if set up in the 
shade, and it will throw off rain; but in the sun it will require a thick 
mat as an additional protection on or preferably stretched above the roof. 
