| 30 | —.06 | —.002 | +.02 |
| 40 | —.07 | +.007 | +.09 |
| 50 | —.07 | +.016 | +.14 |
| 60 | —.06 | +.014 | |
| 70 | —.04 | +.028 | |
| 80 | —.02 | 0.026 | |
| 90 | —.01 | +.017 | |
| 100 | 0 | 0 |
This is illustrated by giving the differences as found, (1) by Rowland, (2) by Guillaume, (3) by Wiebe, between a Kew thermometer and the air thermometer. It is clearly important to establish in England a mercury scale of temperatures which shall be comparable with the hydrogen scale, and it is desirable to determine as nearly as may be the relation between this and the existing Kew scale.
I am glad to say that in this endeavor we have secured the valuable cooperation of Mr. Powell, of the Whitefriars works, and that the first specimens of glass he has submitted to us bid fair to compare well with the 16"'. Another branch of thermometry at which there is much to do is the measurement of high temperature. Professor Callendar has explained here the principles of the resistance thermometer, due first to Sir W. Siemens. Sir W. C. Roberts-Austen has shown how the thermopile of Le Chatellier may be used for the measurement of high temperatures. There is a great work left for the man who can introduce these or similar instruments to the manufactory and the forge, or who can improve them in such a manner as to render their uses more simple and more sure. Besides, at temperatures much over 1000° C. the glaze on the porcelain tube of the pyrometer gives way, the furnace gases get in to the wire and are absorbed and the indications become untrustworthy. We hope it may be possible to utilize the silica tubes shown here by Mr. Shenstone a short time since in a manner which will help us to overcome some of these difficulties. Here is another subject of investigation for which there is ample scope.
So far we have discussed new work, but there is much to be done in extending a class of work which has gone on quietly and without much show for many years at the Kew Observatory.
Thermometers and barometers, wind gauges and other meteorological apparatus, watches and chronometers and many other instruments are tested there in great numbers and the value of the work is undoubted. The competition among the best makers for the first place, the best watch of the year, is most striking and affords ample testimony to the importance of the work. Work of this class we propose to extend.
Thus there is no place where pressure gauges or steam indicators can be tested. It is intended to take up this work, and for this purpose a mercury pressure column is being erected. Bushy House from basement to eaves is about 55 feet in height. We hope to have a column of about 50 feet in height, giving a pressure of about 20 atmospheres; it
