12 L. CASELLA'S CATALOGUE 



30. Casella's Iiisulated Solar Maximum Radiation Thermometer (fig. 30, p. 11), as 



first arranged by him agreeably to the suggestions of Sir J. Herschel (see 

 'Admiralty Manual of Scientific Enquiry'). In this arrangement, the 

 thermometer being in nearly perfect vacuo, the maximum registration of the 

 heat of the sun's rays is obtained, divested of the influence of vapour or 

 passing currents of air. Indeed, this is the only form of thermometer suitable 

 for making comparable observations on solar heat . . 150 



30* STAND for the above, as described by Mr. Stow (fig. 30* p. 16) . 110 



From an admirable series of experiments by the Rev. Fenwick W. Stow, of Hawsker, near 

 Whitby, on the principle and action of this instrument, important improvements have been 

 made, by which tmif ormity of action is secured, and the indications of temperature thus obtained 

 rendered perfectly comparable. (See following interesting description by the Rev. Mr. Stow.) 



The insulated solar maximum thermometer, usually called the black bulb in vacuo, is a 

 sensitive maximum thermometer, having the bulb and a given portion of the stem covered with 

 lamp-black, the whole being enclosed in a glass tube fig. 30, p. 11, from which all air and 

 moisture have been removed, so that the heat of the sun's rays are thus obtained, divested of the 

 influence of vapour or passing currents of air. In an extended series of experiments with a 

 number of these thermometers as usually made, Mr. Stow found that when the stem within the 

 large bulb was not properly blackened, the bulb lost much of its heat by induction, and that the 

 indication of different thermometers so varied as not to be fairly comparable with each other- 

 Mr. Stow also recommends that a stand like fig. 30*, p. 16, be adopted, and that the following 

 rules should be observed : 



1. Place the instrument four feet above the ground, in an open space, with its bulb directed 

 towards the S.E. It is necessary that the globular part of the external glass should not 

 be placed in contact with, or very near to, any substance, but that the air should circulate 

 round it freely. Thus placed its readings will be affected only by direct sunshine and by the 

 temperature of the air. 



2. One of the most convenient ways of fixing the instrument will be to allow its stem to fit 

 into, and rest upon two little wooden collars fastened across the ends of a narrow slip of board, 

 which is nailed in its centre upon a post steadied by lateral supports. 



3. The maximum temperature of the air in shade should be taken by a thermometer placed 

 on a stand in an open situation. Any stand which thoroughly screens it from the sun and 

 exposes it to a free circulation of air, will do for the purpose. 



4. The difference between the maxima in sun and shade thus taken is a measure of the 

 amount of solar radiation. 



The Rev. Mr. Stow will be glad to receive from time to time copies of observations made 

 according to these rules. 



32. Hello-pyrometer, as arranged by T. SOUTHALL, Esq., at his observatory, near 

 Birmingham, by which the following extraordinary results were obtained : 

 July llth, 1859, maximum temperature of air 87 in the sun 216 

 12 89-1- 231-5 



13 ,. 80-5 217 



