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Casella's Patent Mercurial Minimum Thermometer, 1861. Creator: Unknown. Casella's Patent Mercurial Minimum Thermometer, 1861. '...a new species of thermometer lately invented by Mr. L. Casella, Hatton-garden...This instrument...appears to be very serviceable in practice as well as faultless in principle...Its peculiarity consists in a small side tube (a b c), of somewhat wide bore, which is connected with the capillary tube of the thermometer. This side tube breaks off abruptly into a smaller-sized tube at b, and soon afterwards terminates in a bulb. In order to set the instrument it is slightly inclined until the mercury in the side tube be brought to b, where the abrupt change of diameter takes place. This operation may, if necessary, be facilitated by tapping, and when it is finished the instrument must be placed in a horizontal position...We are thus furnished with a minimum thermometer in which the liquid employed is mercury, and in which no separate index is necessary. From a short trial which has been given at Kew Observatory to one of these instruments, there appears to be no practical objection to its arrangement'. From "Illustrated London News", 1861. (KEYSTONE/HERITAGE IMAGES/THE PRINT COLLECTOR)