164 G. S. fsham — Regi8Urivg Sola/- Radiometer, etc. 



Take a strip of glass or transparent celluloid, lav off from 

 a base line which is marked 0° C. 10'U m,n , and make another 

 mark and number it 5° C, for this distance is double the num- 

 ber of difference in millimeters of mercury of pressure of alco- 

 hol vapor, between 0° and 5° C, and so on all the way up and 

 down the scale from the zero, taking the numbers from tables 

 and interpolating by graphic method, values for degrees not 

 given. Having made the scale, lay it on the card and put for 

 every reading you wish to make the number representing the 

 degrees temperature shown by the thermometer for that hour 

 over the base line ; then where the curve crosses the scale is 

 the temperature of the black bulb at that time. 



In order to compare the values of the reading for the same 

 day at different places, I would suggest the following : the 

 paper I have used is ruled in square millimeters and centime- 

 ters. It can be bought in any of the shops and serves the pur- 

 pose ; all that is necessary is to cut it in strips of the requisite 

 width and mark a few of the hours on the transverse lines. 



When reading the actual temperature by the scale for q\^y\ 

 half hour, hour or two hours, according to the accuracy de- 

 sired, make a mark on the hour line at the intersection of the 

 millimeter line corresponding to the number of degrees of dif- 

 ference between the shaded and black thermometer; e. g. if the 

 difference is 12° make a dot at the intersection of the 12th 

 millimeter line (mensured from the base) with the hour line 

 being measured. When they are all measured, join all these 

 dots, and the area of the surface included between the base, and 

 the irregular line so made, is the measure of the local insolation. 



The thermograph should be arranged to trace its curve on 

 the other side of the base line, one millimeter for each degree, 

 or the curve may be plotted ; then the area of the surface in- 

 cluded between the two irregular curves so made is computed 

 by inspection or by a planimeter; the number representing 

 this area is purely arbitrary, but gives a good method of com- 

 paring any number of cards of the same or different places. 



I think there are no corrections to be made for instrumental 

 errors, other than mechanical ones, which is the advantage in 

 using a saturated vapor, except in case of sudden change of 

 temperature, when, though the machine acts almost instantane- 

 ously, it is slower than the actual change, but compares very 

 favorably in rapidity with ordinary thermometers either mer- 

 curial or alcoholic. 



The many ways of varying the machine will be evident upon 

 a little reflection. The form as shown is simply the one that 

 seems to me to be best; a better way of exposing it might 

 alter its form greatly and simplify it, but I do not know of 

 one which will protect it from wind and rain. 



