Examine carefully also your _non-instrumental observations_. See whether you can draw any general conclusions as to the greater prevalence of cloud, or of rain or snow, in one month than in another. Did the last month have more high winds than the one before? Or than the average? Were the temperature changes more sudden and marked? Was there more or less precipitation than in previous months? CHAPTER III. ADVANCED INSTRUMENTAL OBSERVATIONS. The instruments for more advanced study are the following: _maximum and minimum thermometers, wet and dry-bulb thermometers, sling psychrometer, standard barometer, thermograph, barograph, and anemometer_. [Illustration: FIG. 7.] =Maximum and minimum thermometers= are usually mounted together on a board, as shown in Fig. 7, the lower one of the two being the maximum, and the upper the minimum. In the view of the instrument shelter (Fig. 2), these thermometers are seen on the left. The minimum thermometer, when attached to its support, is either exactly horizontal or else slopes downward somewhat towards the bulb end, as shown in Fig. 7. These instruments, as their names imply, register the highest and the lowest temperatures, respectively, which occur during each day of 24 hours. The maximum thermometer is filled with mercury. Its tube is narrowed just above the bulb, in such a way that the mercury passes through the constriction with some difficulty. As the temperature rises, the mercury, in expanding, is forced out from the bulb through this narrow passage. When the temperature falls, however, the mercury above this point cannot get back into the bulb, there being nothing to force it back. The length of the mercury column, therefore, remains the same as it was when the temperature was highest, and the instrument is read by observing the number of degrees indicated by the top, or right-hand end, of the mercury column upon the scale. After reading, the thermometer is set by removing the brass pin upon which the bulb end rests, and whirling the instrument rapidly around the pin to which its upper end is fastened. By this process the mercury is driven back into the bulb, past the constriction. Care must be taken to stop the thermometer safely while it is whirling. After setting, the reading of the maximum thermometer should agree closely with that of the ordinary or dry-bulb thermometer.