232 THE GARDENER. [May 



This being the case, of what practical value is all Mr Hammond's specious 

 reasoning about the particles of water rolling and tumbling down the inclined 

 plane, like so many boulders tumbling down the " Cumberland Screes ? " The 

 great mistake he falls into is in treating the question as one of pure hydraulics, 

 whereas it should be treated as a question of hydrodynamics. I will only notice 

 one other point, and that is about the back motion of the water in the flow- 

 pipe. He makes a great deal of this ; but he is again labouring under a very 

 serious mistake in assuming that this motion goes on until all the water in the 

 flow-pipes becomes of equal temperature. Some hot-water engineer seems to have 

 admitted this to Mr Hammond ; but it must have been some one who knew 

 very little of what he was talking about. The back-motion in the flow-pipe 

 must immediately cease as soon as the average temperature in the flow-pipe be- 

 comes higher than the average temperature in the return. The difference of 

 temperature between the upper part of the flow-pipe and the under part has noth- 

 ing to do with it, any further than that the upper strata travels faster than the 

 colder under strata. It is well the question should be discussed ; but I fear deep 

 stokeholes are a necessary evil, which must be borne with on account of pathways, 

 smoke-flues, and other causes. 



A. D. Makenzie. 

 2 Grove Terrace, Edinburgh, 

 March 15, 1879. 



THE PENTSTEMOK 



I do not think the Pentstemon is quite so popular as are some other 

 flowers of the florist class,— at least, in looking round gardens, it is 

 not so conspicuous as it well might be. The fact that this flower 

 is liable to be destroyed in ordinarily severe winters may account for 

 its absence from so many gardens ; for although the dibbling in of a 

 sufficient number of cuttings in autumn to keep up stock may appear a 

 simple enough affair on paper, it may become in reality one of the 

 proverbial straws which break the camel's back, and, naturally, this 

 particular straw is not put on. Of course, nurserymen have always 

 plenty of young plants with which to supply customers ; but here again 

 the inevitable steps in, and Pentstemons are omitted from the list of 

 cultivated flowers. However, there are sure to be some of Flora's 

 wooers suing for favours with regard to this particular flower, and to 

 such these cultural notes may prove somewhat acceptable. The florist's 

 Pentstemon has been brought to a high state of perfection within the 

 last decade — continental as well as home cultivators contributing to 

 bring about this result— so that the number of really fine kinds is 

 somewhat perplexing in selecting sorts for a collection. I have not 

 found the newer sorts, as a rule, more worthy of cultivation than kinds 

 of older date, and, accordingly, would recommend these latter to be 

 drawn on in forming a collection, adding, of course, kinds of appar- 

 ently superior merit as occasion may present. A few select varieties 

 pay better in all respects than a large collection of sorts, many of 

 which eould be usefully relegated to obscurity. Now as to the various 



