45, CORNHILL, E.G., AND 122, REGENT STREET, W,, LONDON. 77 



The Summer and Autumn of the year 1859 were specially remarkable for 

 a most unusual Thermometric and Hygrometric condition of atmosphere, and 

 Londoners will long remember the state of the River Thames during that period. 

 Deficiency of water supply during 1858 and 1859, and great evaporation 

 (often to fourteen degrees of thermometrical difference in Mason's Hygrometer), 

 caused a condition of its stream excessively offensive, if not actually pestilential 

 and unhealthy. Everywhere a want of water was felt, and this had been 

 of considerable duration. In August the heat reached 92 (in places where 

 usually summer heat is not above 80 Q ), and the temperature of evaporation 

 was 78, by the same hygrometer. 



INSTRUCTIONS FOR THE USE OF THE WET AND DRY BULB 



HYGROMETER. 



102. The muslin on the bulb of the Hygrometer should be washed 

 occasionally by pouring water over the bulb ; and it should be replaced by a 

 fresh piece at least once a month. Accuracy depends very much upon keeping 

 the wet bulb clean, free from dust, and not too wet. 



When the bulb is frozen, some cold water should be taken from nnder 

 ice, being cautious to raise its temperature as little as possible, and the 

 thermometer bulb should be wetted with it by means of a camel-hair brush or 

 feather. After waiting a few minutes, the temperature of evaporation may 

 be observed. The water should be either distilled or rain water, or if this be 

 not procurable, the softest pure water which can be had. The water vessel 

 should be replenished after, or some little time before, observing ; because 

 observations are incorrect if made while the water is either colder or warmer 

 than the air. 



In connection with the barometer, the Wet and Dry Bulb hygrometer is 

 very useful, not only on land, but especially at sea, where other kinds of 

 hygrometers cannot be practically used. A fall in the Barometer is indicative 

 of coming wind or rain ; if the hygrometer shows increasing dampness by 

 the difference of the readings becoming smaller, rain may be anticipated. 

 On the contrary, if the hygrometer shows continuing or increasing dryness, a 

 stronger wind is probable, without rain. 



The Hygrometer is eminently useful in regulating the moisture of the 

 air of apartments ; a difference in the thermometer readings of from 5 to 8 

 being considered healthy. Many diseases require that the temperature and 

 humidity of the air which the invalid breathes should be very carefully 

 regulated. In a room, the hygrometer should be placed away from the fire, 

 but not exposed to draughts of air. 



